That is part of the big problem. Women are such better friends with their friends than guys are with theirs in many ways, that they turn on the friendcharm with guys, who interpret it as flirting. The guy then gets shot down. The next woman who acts really friendly then is viewed with definite suspicion.
WORD.
I just so happen to love flirting with waitresses. You can have all the fun you want and there’s no risk of leaving her feeling led on. Risk free flirting.
But this girl eally wasn’t giving him much to work with. She was friendly, but what is he supposed to say? “Yeah, I do my algebra homework there. Doing anything tonight?” This is a kid who has women all over him. Some waitress recognizes him and he’s clueless for not flirting with reckless abandon? Maybe he just wanted to eat lunch with his mom and sister.
Look, if it makes the OP feel any better, up until I was 15 I had zero interest in women. I knew I wasn’t gay, but I just wasn’t interested in women at all like that.
Then, one day when I was 15, it was like a switch flicked on, and I was most definitely interested in women. So don’t fret because your 14yo missed what may or may not have been an obvious flirting attempt. He’ll come right soon enough. 
I think your son responded perfectly appropriately, myself. Just because others have displayed an interest (beyond friendship, perhaps) in him doesn’t mean he automatically must decide whether or not he has an equivalent interest in them,particularly on the spot when first meeting someone new (and with his parents in tow!). He’s being friendly and polite by what you describe. I’d even say he’s being respectful of the girls as human beings in not assuming that these girls must have the hots for him if they want to spend time with him.
He’s not leading Katie and the others like her on, either. They’re interested in him? OK, then it is up to them to make their interest clear enough if they want to find out whether he wishes to reciprocate or not, or just wants to stay friends. It’s good for Katie to learn how to be brave enough to put her feelings out there and accept the risk of being shot down (and then learning how to handle that). It’s also important for Katie to learn that if you don’t make your wishes more apparent (or aren’t willing to take the risk of the same), then you have just about zero chance of getting what you want. She can sigh and enjoy being in the throes of unrequited "love"with a very nice 14 year old boy, instead, as the consolation prize.
Amen, brother.
I am 41 as well, and my radar has always been out of whack.
All my GFs in High School always approached me (and by approached, I mean usually gripped me in a lip lock at a drunken party or something like that). I could never approach women.
Later, while stationed in the army in Germany, I was accused of molesting a female patient. (I didn’t do it, and the following investigation by CID proved that she not only lied, but confessed to trying to get a payoff from the lawsuit which would come next. I was a convenient target because I usually worked the night shift). During that time, I couldn’t get a date to save my life, regardless of what kind of vibe I put out there.
The only reason I found my next GF was we were friends for about a year during this event. She told me (later) that she was trying to get me to make the next move almost the whole time. It was only when she took off all her clothes and asked me what I wanted to do next did I get a clue.
She is still a good friend.
All that as a pretext, I must admit that I am neither Adonis nor Quasimodo, but I do have my good qualities. They are just not in the ‘looks’ department.
Just like you would do in any small-talk situation; just make small talk. “Yeah, I was in that class. That professor was a real snore; I’m glad I’ve graduated now and working at Megacorp Inc.” It give the other person lots of conversational hooks to hang a continued conversation on. If the girl (or guy) is just being polite, the discussion will probably end with, “Well, it was nice seeing you again. Bye!” If they are actually interested, the discussion might continue more like, “Yeah, that professor was terrible. I barely made it through that class. So, you say you’re working at Megacorp, Inc. How do you like that?” So the conversation continues and expands.
I’m a guy. I’m still pretty much clueless at 30.
Having said that, I must admit that I’ve started to get a clue, sorta. I’d say I’m in the incipient stages of getting a clue, though things may never really come to fruition. I think I started to get a clue in the last 5 or 6 years.
I finally started to believe people when they told me I was attractive or when they told me a girl was flirting with me. I think one of the reasons this came about was spending time with my female friends and acquaintances and becoming familiar with their tastes in men. I started to see the amazing variety of people they were attracted to–real, honest-to-goodness pants-wetting attracted to. I also saw how often they were afraid to say anything and thought that the guy probably wasn’t interested in them and so on. I started to realize that probably there were girls who had had that reaction to me and I didn’t even know it.
I still couldn’t say I know when a girl is expressing interest, but at least it seems like a possibility that she is.
(I sometimes think that I have built up an unconscious repertoire of ways to deflect a girl’s interest, not to mention a repertoire of excuses for why they are not interested in me. I wish this were not so, but I have a sneaking suspicion it is.)
Thirded.
Plus chicks are weird. I remember in high school getting “obvious” signals and then when you try to move in for the pick up, she says that all she wanted was to be “friends” and makes fun of you for hitting on her.
Either that, or you son’s a homosexual (not that there is anything wrong with that) JK
As to the Title Question: Oh, we get plenty of clues.
They are all encrypted and we have to figure out the key by blind trial and error.
Makes life more of a challenge, and someone keeps telling us that is something to be desired.
Me, I long ago decided to just assume “she’s just being friendly” or at most “she’s just play-flirting for fun” no matter what, in part because those who are just doing so IME tend to get far more upset if I misinterpret ulterior motives, than the other way around.
But a boy at 14/15? I’d think he’s not expected to even be able to recognize a clue if it painted itself purple and danced on top of your table spelling out C-L-U-E to the tune of YMCA, and he has license to enjoy that freedom from expectation.
Unless he actually wants to avoid starting a spiral of conversation – sometimes the teenage mind will just slide into into solipsistic mode, and the thought process becomes “Oh, that’s some Mundane Pointless Stuff You Feel Like Sharing. Oh joy - not.” Socially uncouth, yes, but listening and responding to another human’s MPSIMS every single time in case there is important information for you amidst the chatter is a social skill that one has to learn, one’s not born with it.
Also, as others have mentioned, in the specific OP circumstance it would be entirely within the scope of the teenager mind that the last thing you want to do is continue and expand a conversation with a peer with the parents around. The very existence of their elders is sometimes mortifying enough, never mind their presence.
And I must agree with serious lark that there is no harm or foul done if others are interested and he merely does not respond in kind – he’s not being rude and he’s not “leading on”, he’s just responding HIS way. Heck, that situation with genders reversed is considered perfectly par for the course and something he has to live with.
OF COURSE, yes, he may be showing early warning signs of the dreaded Mister Oblivious syndrome. Y’know, from what’s been discussed so far it seems he would not be in limited or unpleasant company were he to be a part of our cohort. Seems to me the ladies are fully aware of the prevalence of the Obliviousness Disorder among us and are willing and able to take the necessary steps to compensate, as Uncle Brother Walker discovered 
I had the same thing happen to me.
Apparently women are not as into clean clothes as society and our mothers would have us believe.
I wash and folded em all and I barely got a thank you…
56 here, and I can never figure out why any woman would be interested in me. So of course I never notice when they are. I’ve been to parties and was later told that some woman there had been hitting on me all night, and I was totally oblivious. My late wife had to use a clue-by-four repeatedly before I got the hint.
That still doesn’t give me a clue. I still haven’t any idea when it progressed from “it’s just a conversation with someone” to “she’s interested in me.” The girl’s half of the conversation is completely innocuous.
The only thing which might be a hint here is “this girl is continuing to talk with me. I wonder what that means?”
In this situation I would be likely to find the interpretation “she’s probably making small talk because it’s her job” and not analyze it further.
yeah women in any kind of customer service job are just flat out off limits as far as I am concerned, it just adds a huge level of confusion to an already confusing situation.
I wish females noticed me enough like that for me to realize I need to get a clue…if that makes any sense.
Okay, so I can usually pick up on subtle hints but sometimes I have no idea if there is a flirtation/interested connotation behind it. Like for example, there was a girl at my workplace who would tease me a lot and we sort of had a friendly antagonistic relationship going on (she’d make fun of me, I’d make fun of her). One time when we were on break together while we were making small talk she directly asked me how old I was. Now, was she asking because she was interested in me? Or was she just making small talk? I couldn’t tell you.
Of course, I’m also horrible at small talk which could be one reason I’m not that good at picking up signals. If I was better at small talk I would have more exposure to such signals, I think.