i know your son has ADD (or adhd). one of mine has a mild case of it too. i think he's grown out of it, but school was always hard for him. i'm gonna sound like one of those commercials for those learning centers, but yelling and screaming didn't work. neither did punishment. i used to think it was just him being lazy but it was more than that. the most you can do is CONSTANTLY work with him and stay on top of everything. be in contact with his teacher as much as possible. Its going to feel like YOU'RE the one in school, and I don't doubt that you've probably said "who's in school here, me or you??" but its the only way you can assure than he does his work and tries, because without you - its easier just to goof off and let it go.
my other child, btw- i have never had to check her homework. its always done, she's always studied and she's upset if she doesn't get 100 on a test.
My other child, as you know, i didn't raise myself- he gets good grades most of the time without trying, but doesn't care if he fails a test. its frustrating because I know he could get straight A's if he tries, but he gets mostly A's with a few scattered C's and is happy with that.