Nomad, the first thing I started thinking about when I was reading your post---just catching up here---is it sounds a whole lot like "this generation." I realize you said he has other stuff going on, and I guess you can attribute some of it to that as well, but in general, I think most of us have overindulged our grown kids, and they have had basically everything they wanted, so...they expect a lot.
I know you said he is a very high achiever---which is great! Perhaps giving so much is keeping him "as a kid" instead of him feeling like a grown man. I do think we have to resist our natural temptation to want to help. Helping keeps people back, not really helps them, most of the time (not all the time).
I agree with pulling back on your generosity and instead of just acting to fill in any stated gaps, keep your "gifts" few and far between.
Maybe it's about creating a more reciprocal and balanced relationship. If he's not the best person to ask to get the extra ice or wait at your house for a repairman, then okay, but there needs to be some "give" on his side, whatever that looks like.
I wouldn't overthink this, just, if you are getting a bad reaction to one thing or another, like you mentioned, change your approach and see what else might work.
I agree that I don't like the "bite me" thing. You've told him now about that. Next time he says it, just go silent. No need to say anything and no reaction. Back off at that instant. That might be enough to put a stop to it. Sounds like he is being a bit "flip and funny" and you're right, it's not.
husband and I were talking last night about Difficult Child and how much progress he has made in the past year. husband said, now, he needs to step up his game and quit wearing DG ball caps and keep his pants pulled up. Same type of thing...have some personal dignity, is what I call it. Grow freakin' up. It's time.
First rule of the forum: stop the flow of money. Take it to a mere trickle. That's always always good advice.