Not sure what it is, but it almost seems like he is retreating from his gifts or is bothered by them.

He is very good at chess and got a lot of attention for it last year, especially from coaches and other adults in his life. He began to see that he was unusually good at this thing. He has now retreated from chess. He insists he still wants to play, but seems to be studiously avoiding it, regardless of who asks him to play (that is, doesn't seem to be a case of not wanting to lose). He chose not to attend chess camp after begging to go earlier in the year. This is very surprising coming from someone who used to eat/sleep/breathe chess. It could just be that a phase is over--but...

He recently also became able to read many of the same books as his older sister (10), including Harry Potter. He really enjoyed these books, but when I got him some other books around this level he read them for a while (got 50 pages in or so, so didn't just get bored right off)and then told me they were too hard and boring and retreated to much easier books he was reading long ago. He does not want to even look at serious chapter books right now.

Drawing skills/penmanship have also kinda gone downhill, but that's not as noticeable and may not be anything.

He's about to finish up kindergarten with a teacher he really loved. Could that be it? Fear of getting older? Could we be putting too much pressure on him?

If I try to talk to him about all of this, he bursts into tears and says he doesn't want to talk about it and I'm making him feel bad---agh! So he is very sensitive and angsty about whatever this is. There is something there.