S.B.
I also have an 8 yr old with ADHD (severe, combined type) as well as other dx's, and he is on meds as well. We don't keep any sugar treats in the house - just healthy treats (grapes, carrots, etc.) so that eliminates the "sneaking" food. The temptation is just too great for these kids and I was tired of finding hidden wrappers and getting mad about it. If chocolate milk is the only "treat" he is getting at school, then I would let that one go. I would talk to the teacher, though, about the treats that the kids get when they do a good job. I actually supplied the teacher with a big bag of pencils, erasers, stickers, etc. for about $20 and asked that she give these items out to the kids in the class, rather than using candy as an incentive. She was thrilled to do it!
Finally, look for a "trigger" food if you are still concerned about diet/ADHD. For one kid it was milk, another it was peanut butter, and another it was ice cream, and yet another kid it was cough drops (crazy, I know). Not all of my kids were/are ADHD, but their behavior would go downhill have these certain foods.
Hope at least one of these ideas helps!