Maybe you need to look more into what he means by "giving his brain a rest"? It could be that he needs it to have a rest from something other than the schoolwork and that having something more difficult to concentrate on would be restful.
I'm thinking about my DD12 and what I learned from her last spring. Apparently for years she's had a party of negative self-talk going on in her head during school. This abruptly disappeared when, for a time, school became challenging and required all of her attention. (And that's when I learned about it, when it went away.)
Anyway, it's possible that he's unhappy that he thinks he has to do more work than other kids, but it doesn't sound as though he is working harder than the other kids. It sounds as though he is coasting. Sometimes lack of work can be tiring all by itself. Difficult to get out of that position though.
I agree, kcab. That's why I suggested what worked for our GS10 in that situation. Running until he is winded seems to invigorate him; same thing with solving a really challenging logic or Soduko problem. Boredom can be very draining, and a 'rest' seems to make it worse.
