Ah, that makes sense. Of course, he does not work in my same office, I talk to him infrequently, and the subject of his Jewish-ness (or my atheism, for that matter!) has never really come up. 
But, I like your perspective, Shot.
Coming from a family with a mix that includes moderately observant and nonobservant Jews, and non-Jews, I’d say that the main courtesy is to avoid assuming that the person is Christian. “What is your family doing for Easter?” gets old (not offensive, though) when you don’t do Easter. On the other hand, I agree with posters above that it’s no particular courtesy to assume that someone is observant. Neither I (non-Jew) nor my husband (nonobservant) really enjoys explaining that no, what we’re eating would not be suitable for Passover for an observant person, but that we don’t care. It’s OK if it’s pure curiosity, but every now and then you get the feeling that someone is oddly annoyed that you’re not more Jewish than you are. If there’s a practical issue, like bringing food to something or planning a Saturday event, then it’s good to just ask.
So, really, it sounds like if it’s important enough to my boss that he brings it up in casual conversation, then it makes it more appropriate for me to acknowledge.
My 2cents.: Unless the guy’s wearing a yarmulka, I’d lay off the “good shabbas” part, unless you’re tight with him. The same goes for all expressions used in Yiddish or Hebrew, unless you want to show him, in a friendly way, that you know what’s going on. (Hell, religious Jews say “Have a sweet new year”; even-more-religious Jews say “May you be inscribed/sealed in the book of life,” depending on what week of the holidays it is is. Try that on for size.)
Rosh Hashanah is the biggest deal for almost everybody, as is Passover. On Rosh Hashanah “Happy New Year” is clear and nice (it feels weird at first, because to you it ain’t). A point: if someone says back “I don’t observe the holiday” no harm done. Most truly secular or even atheist, ethnic Jews like the respect now and then.
Purim really is a fun time, and Happy Purim is terrific. But this one, sadly (from my Jewish point of view), often goes right by most Jews without their even knowing it. But if they have children–which the holiday has come to be about–a dreidel (top) or grogger (noisemaker) are standard cute gifts. If you think they care, have at it.
Yom Kippur is the one that stumps non-Jews the most, I believe, as well as most so-so observant Jews or completely secular ones for whom the only time they give a damn for holidays is Yom Kippur, when after a year of ignoring this kind of stuff, they go full out and fast for 24 hours.
You can’t say “Happy Yom Kippur.” For starters, it’s not a happy day. The religious Jew is about to bust his balls standing in front of the King of Kings all day pleading for his life and the lives of all Jews and everybody else on earth, which is an ordeal, to say the least. If he’s secular and is fasting for any number of reasons, he certainly will feel like shit.
The common thing to say before Yom Kippur–and one that always sounds well meant and sympathetic from Jew and non-Jews alike, is “Have an easy fast.” If the the guy or woman starts laughing and says “I’m going to pig out” or some such, a) he’s taking advantage of a free religious holiday and is making fun of you for falling for that whole Jew thing, making him an asshole, or b) he is really throwing back friendliness in your face, making him an asshole (A similar case would be that after you give a “Merry Christmas,” he responds with somethin like “Christmas is only for dupes of the mass-marketing blah blah and you’re such a fool for even thinking/saying that”: asshole.)
Passover is the easiest: Happy Passover. Hannukah is also the easiest: Happy Hannukah. These are like Christmas and New Year, so to speak, and are kind of important.
To me, in this blessed USA, it feels warm that non-Jews care enough to realize significant life events of Jews.
I will join the choir of those saying you should be careful not to embarrass him by pointing out holidays he might not be observing. I once learned a pretty close friend was Jewish. The first thing that went through my head when she said she was Jewish were the too many plates of shrimp we had shared on saturdays.
As someone said, being aware of the date when they fall is a big thing to do. To show that you are aware, a simple “how were the holidays?” or “what have you planned for the holidays?” should be enough until you get a good grip of what he observes and what he doesn’t.
That said, I applaud your interest in learning about his culture. By all means read up on the subject and broaden your knowledge. Just make sure you are not outdoing him when you talk with him.
Yep. It’s not about earning points for myself. It’s about being aware.