I assume you have talked with the teacher; is there a school principal who can also advise you? A school counselor?
Do you have the option of private school, or another school setting where he is allowed more freedom in a setting with fewer kids?
I met with the teacher and the assistant principal and we agreed on tracking each daily activities on a piece of paper and grading him accordingly. Also we agreed on afternoon nap since most of his problem always happen right after lunch.
As for a private school mentioned by RockRose, I think that will be my plan B.
An afternoon nap? does he normally nap? And he can take a nap at school?
the fact that you have identified when his problems are happening is very good. Does the school do something different after lunch? Any chance that you could be there to watch what is going on?
Have you taken a look at his sleep patterns at home? Is he still going to bed on summer hours, but waking up on school time. Lack of sleep can really mess a kid up.
Anyway, don't give up just yet. Also, make sure that you don't discipline him at home for his "grades" at school. They should only be used to let you know how he is doing and where his problem areas are.