I would sit down with him and make a giant list.
1) Triggers. Things that really set him off.
2) Things that soothe and calm him.
3) Foods that seem to make the problem worse.

I found with my son that a gluten/dairy free diet really helped... mainly because the body expends so much energy digesting those foods that its less energy available for regulating the nervous system. And load up the protein/lessen the carbs.

Our OT had some great advice in how to frame it.. there is a sensory bank account in your sons body.. social situations, transitions etc take away from that bank account.
Exercise, deep pressure, calming music, meditation, put deposits into that account.
You just need to be able to have the strategies at hand to make sure you don't get "overdrawn".

Does he have a physical outlet of any sort? That would help.
Perhaps headphones with music? Or ear plugs/sunglasses while at school to help limit the sensory load?

L- Theanine supplements help.

I have also had amazing success with CBD oil.( i have a post in Introductions regarding that).

Good luck!