Lauren
This is really a common question and one that there is no good answer to. While eating less sugar and a more healthy diet is always a good thing, there is no definitive link between diet and ADHD. The links in the other answers will give you some additional information on this subject.
That being said, a small percentage of children have reactions to different foods and if you feel this may be the case, you can ask your physician for a referral to an allergist that would be able to test your son for specific food allergies.
I hope this helps.
Eileen