S.,
Wow, I bet you are just so exhausted! But now you have a starting point and can take control. You now KNOW that to some degree diet affects your son's behavior. Obviously, whatever is put into the body affects how it runs, somehow in modern America we have lost sight of that almost entirely. Unfortunatley, sometimes doctor's focus on treating symptoms of problems, not the root cause.
Now you can make a difference. You can take control. It will be time consuming and not easy, but you can do it. Start doing some internet research. Check out amazon.com for books on the subject. The best book I have read so far on diet/and kids behavior is "Little Sugar Addicts". It really explains simply how certain foods/chemicals alter brain/body chemistry and gives an easy step by step, slow and REALISTIC plan to gradually wean your kids off the bad stuff and onto the good stuff...where the kids learn to take note of how they feel and own their food choices. I have been into nutrition since my son was born, and we try to stay mostly organic/local anyway, but THIS book gave us the missing piece.
I am in the process of implementing the plan in that book for my 4 year old and (even myself) and can tell a difference.
On a side note- I am appalled that a physician, who is supposed to 'do no harm' recommended artificial sweeteners!IMO, they are poison. DO the research!
Best wishes to you and your family.