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 was going to suggest that also, that way he won't be so hungry by the end of the day. Nuts and sultanas and other dried fruits make awesome energy snacks, as do rice crackers with whatever you why on them (avocado cheese and tomato is delicious!!)