Question:
My son is biting his hand. Why does he do this and how can I get him to stop?

My Opinion:
In my experience, kids bite for many reasons. The trick is to figure out which reason/s apply to your son's situation.

First, many kids get aggressive, including biting, because of chemical problems. This would include gfcf, which I think you already know about. Also, phenols are highly likely to cause this problem as well. My list of highly phenolic foods is -- food dyes, tomatoes, apples, peanuts, bananas, oranges, cocoa, red grapes, colored fruits, milk. If your son is eating more of these foods than previously, you might want to consider reducing or removing them to see if his aggression/biting is reduced. Finally, if your son has recently been sick, or if you have just started or just stopped certain medications for him, this may result in chemical issues which can cause aggression.

Second, you should look at his environment. Did he just get a new teacher? Or is he going to a new school? Or are you having difficulties at home? Or a new babysitter? It could just be the transition, maybe he does not do transitions well, or maybe it is confusing and frustrating for him, or it could be that he is not being treated nicely by new people in his life. Is he only biting his brothers, or you and his father and his teachers also? If just his brothers, this could just mean that he perceives his brothers are easier targets, but it could also mean he is for some reason angry at children, you can check that out with his school or wherever else he is with other children. Also, see whether his brothers are intentionally or even unintentionally causing him some stress.

Third, check to see if in some way the biting is being intentionally or unintentionally encouraged. Have other people been smiling or laughing at him when he bit in a small way, so now he does it in a larger way? Does he smile after he bites, or does he seem to bite with anger? Do you think he feels that he is not getting enough attention, which would encourage him to bite as an attention-getting device? For many kids, negative attention is better than perceived insufficient attention.

Last, you do need to let him know that biting to release his aggression/frustration/whatever, is not an appropriate behavior. Ask his teachers, or maybe you already know yourself, which method of behavior modification works best for him. For example, ignoring the behavior, or saying "no", or clapping your hands, or whatever works to let him know that behavior is not acceptable. Also, consider giving him another outlet. Many people tear paper, hit punching bags, scream, whatever. You do not want to encourage a behavior that you will later regret that he has, but he does need some sort of acceptable outlet. Hopefully, you can find the cause of his aggression and remove it, but until that happens, he does need to know that biting is not acceptable. He also needs to have another method of releasing his stress, as there will undoubtably come other times in his life when he needs to release stress, and he needs to learn acceptable ways to do that.