sit down with him when he does his homework. He needs to know that he can tell you if he doesn't understand something or has a subject that keeps his attention less than others. perhaps look into tutor time. Keeping the lines of communication open is key. Talk to his teachers, see what areas he really struggles in
I had to take a non addictive pill all through school even college to help me to better stay focused in school I went from low grades to actually getting honors classes. It's called concerta it has a slow release and isn't addictive an wont slow your child down at the end of the day. Same energy better grades (: look into it, it can really help!