I know you received alot of great advice. I wanted to add in my own personal experience with my 6 year old. Several years ago we noticed behavior issues from High Fructose Corn Syrup. The final culprit was Mott's applesauce. They sneak the HFCS in so many products. He also has had issues with food dyes and we watch for those issues also.
We have just told him and anyone whose house he goes to, his teachers, etc. that he has an allergy. People are very open to helping us watch for things. My son is also very good about knowing what he can have and what he cannot. His pediatrician listed it as an allergy with behavior issues and headaches on his health form for school so the staff is aware of it.
We have seen issues within 1/2 hour and the behavior can continue with outbursts and anger for several days. My son knows the feeling and knows that he has had something with HFCS in it.
I hope you get to the bottom of what is causing issues. Our son's behavior has been night and day since we have monitored his diet. We all need to be aware of what is being put in our foods.
Best of luck! I am sure your son will learn to self monitor things quickly.