But Happy Holidays is a pretty secular greeting (yes, holiday comes from holy day, but very people think that way any more). The festival of Festivus could fall under Holidays after all!
I’d probably answer the silence with a smart-ass response, like “Well, Bah Humbug to you, then!”
But others upthread are right: this person could be grieving, and maybe things just aren’t happy right now. Continued encounters should be met with a smile and nod.
~VOW
Maybe he doesn’t want the pressure you’re putting on him to have a happy holiday? Maybe, just maybe, he’s had 10 happy holidays in a row, and by God, he’s ready for a crappy holiday.
apologies to George Carlin
I had a coworker who was highly religious and had that whole ‘white christians are victims’ complex going on, so I gave him a christmas card that said ‘happy holidays to someone who thinks this card should say merry christmas’. He was not amused.
Who says you have to wait until you’re 80? 
That’s kind of obnoxious. Is there any possibility he didn’t hear you? I’m guessing unlikely, but trying to give the benefit of the doubt. I’m projecting, but sometimes I totally zone out, especially if caught in the middle of doing something. That said, even as an atheist, I always say Merry Christmas if someone says it to me. I’d take the recommendations noted above - if he’s not responding to it, just say Hi instead of Happy Holidays and leave it at that. If you feel comfortable enough to ask you can just say, “Hey, are you okay?”
Just ask him… then tell us!
This guy is driving me crazy… it’s keeping me from having any happy holidays.
My hearing is about as bad as it can be and still benefit from a hearing aid. That said, even with my hearing aid I miss a lot if I’m not concentrating on the speaker, or if there is any background noise.
I’ve had many situations where I’ve discovered after the fact that I didn’t hear a person. People have felt offended, angry, or hurt by my lack of response. The funniest ones are the situations where my non-response is interpreted in a very specific way.
If he has a messiah complex and it was Easter season, I could understand him feeling a little prickly. But, Christmastime? Maybe he’s doubting that whole “mom says she was knocked up by a ghost” story?
I can think of two:
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He is either an atheist or of a religion that does not celebrate any of the December holidays. And he’s being a dick about not acknowledging that these holidays exist and it is a time of year where it is traditional to wish happiness to those around you, and that it is possible to do so without proselytizing. And New Year is a holiday too.
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He is a Christian who is offended by anything that attempts to suppress Christianity because after all we are a nation of Christians established by Christians. And he’s being a dick.
OK, three. I used to work with a JW who would not participate in our monthly office birthday celebration. This consisted of serving a cake and naming the people with birthdays that month. And I won’t say this is a dick move but listen to what this ex-JW had to say.
I would experiment… Next time say Merry Christmas and note the response. Then altermate and see if all are ignored or only Happy Holidays. And then a “Your pants are unzipped” to see if they are even listening.
You could try saying that you hope he has a peaceful winter solstice. Who could argue with that?
You’ve just given him his Christmas present!
Irony alert! User name/post combo.
(You knew this was coming!) 
Not everybody likes Christmas. Maybe the acquaintance was miffed because he hadn’t gotten around to shopping yet, or had to put up with obnoxious relatives visiting, or was thinking about how convoluted his schedule would be, or any number of reasons, and didn’t want to air his grievances after you wished him well.
He’s obviously made other plans for the holidays and doesn’t want to be told what to do.
I’m wondering if at some point he wished you Merry Christmas and you replied back Happy Holidays. That comeback could be interpreted as either somewhat akin to political correctness, or a rejection of Christianity. I can see both being viewed negatively, and someone declining to interact with you based or their interpretation of such a reply.
If someone wished me Happy Hanukah, I’d reply back Happy Hanukah to them, even though I’m not Jewish. Replying back Merry Christmas or Happy Holidays would seem to be verbally diminishing an occasion they care about. It’s a small thing to be offended about, but some people are sensitive.
I find that “Right back atcha, slick” is the perfect one-size-fits-all reply for overly-specific greetings and well-wishes that I cannot in good conscience reply to on par.
That is by far the best choice. For whatever reason he doesn’t respond, don’t pick at it. Life is too short to worry about a greeting being snubbed.
You must be awfully young yet. When you get a little older it will be “No, another year has flashed by so quickly”.