Could that be it? Fear of getting older? Could we be putting too much pressure on him? (I have completely let the chess thing go, although I know his chess coaches at school are going to be upset if he quits!) Could someone have said something to him that made him feel like an outsider/weird?
I think it could be any or a combination of these, or it could be something else you might not even expect. My advice would be to let him set aside chess for now, and enjoy summer vacation. Once he's more relaxed, he'll probably tell you what the problem is.
My DD9 went through a similar process with literature, then went back to more difficult literature later. We gave her the space to read whatever she wanted. That may have been related to a fear of getting older, because she has DEFINITELY communicated to us loud and clear about not wanting to grow up, and the loss of certain things associated with childhood. To counter that, we emphasize the positive aspects of getting older whenever we get the chance. Yesterday she got to roam a small waterpark with just an age peer, and she loved the autonomy.
She went through a long period of rejecting the guitar, which was weird, because she loves it. It turned out that she was having difficulty reaching the top string, and she decided she just couldn't play. When she finally explained that to me, I showed her how I adjust my grip to reach the same string on my classical guitar (it has a wide neck), she copied it, and away she went. She was avoiding the conversation because she was afraid I'd be upset that I'd wasted all that money on her good guitar, and she couldn't play it.
