can I also add the food he is eating at school is all high gi so it will not fill him up.

Maybe it is worth changing what he is eating during the day to give him sustained energy/blood sugar.
i actually think most of that stuff is low gi? the yoghurt i give him is easiyo, its probiotic, thats low GI, and i make him sandwiches on wholemeal bread. also the muffins are usually fruit muffins with low sugar....

meant to say i sometimes add carrot sticks in there as well...also what i pack for recess, he doesn't have to eat it all at recess, he can eat the yoghurt and/or mufffin or whatever with his sandwich.

but thanks for the advice ladies, some good suggestions there.