Much has been written over the years about the affect of diet on ADHD. It can be a challenge to help children with ADHD and no treatment (medication, diet, supplements, etc.) works for or is right for everyone. There was recently an excellent review article about this topic in the 'Pediatrics' journal February 2012 issue. It was written by Dr. Millichap and Nurse Practioner Yee of Children's Memorial Hospital in Chicago.
To summarize the big picture of this topic: there are options to try with supplements or modified diets; they work for some but not all children; it is hard to predict who they work for and who will tolerate the diet; overall side effects are rare with supplements or modified diets; and most children needed prescription stimulant medications to control their ADHD symptoms.
Omega-3 and -5 Fatty Acid Supplements: Probably the easiest to try and certainly could help. Dose: Nordic Gummie Bears (3 per day) or Nordic Fishes Chews (3 per day) or Nordic ProEFA (2 seoftgels per day) or Nutrigold Softgel (1 per day). They should be used for at least 2-3 month, longer if needed.
Feingold Diet or Oligoantigenic Diets: A small group of children with ADHD symptoms respond well to these elimination diets. They are difficult to pull off (months of avoiding milk, cheese, wheat cereals, egg, chocolate, nuts and citrus (Oligoantigenic) or avoiding apples, grapes, lunch meats, sausage, hot dogs, and cold drinks with artificial flavors or colors (Feingold)). You and your child may benefit from meeting with a dietician to follow one of these diets. They certainly require patience and perseverance.
Food sensitivities or allergies: Although a few children respond to avoiding certain foods, allergists will tell you this is not a likely help unless there is a very specific worry that "he is fine unless we eat this food and then things are a disaster". Also, avoiding sugar or artificial sweeteners in children with ADHD only needs to be done if you clearly notice that the child's behavior is worse when they have more intake of these. Otherwise, do not stress these.
Iron or zinc deficiency: Although some studies have shown that children who are low on iron or zinc have ADHD-like symptoms, not enough has been done to help me feel strongly about investigating this (with blood work) or supplementing iron or zinc for the average child with ADHD. More studies are likely to follow in the coming years.
I think this is a good summary of where we stand now with these issues. I hope this is helpful. If you have further questions about this, please discuss it with your physician.