He might just be a sad kid. No labels to explain away his troubles, he might just be sad, or mad or whatever. The solution lies in finding out what the problem is and fixing it. Does he get picked on at school? Another school with a clean slate might help. Does he think his homelife is bad? ETA: Obviously after making sure he does not have dyslexia or some other pinpointable affliction. I just think we have gotten out of control labeling kids just to give them a name.
I would go to a psychologist or therapist and let him talk out his issues.
I am pretty intelligent, and when my mother died my father was shocked to find out I was at best making C'sand D's when I was an A student before their divorce. I was so incredibly sad, thats all, just depressed. I was taking care of my mother, who didn't want her brother or my father to know she had not left the house for an entire year. We were broke and I was considering quitting high school and working at Mc Donalds full time when her severence pay ran out. She died 10 days before I was to quit school. I was not the most popular kid in school, and I was in general not happy.
I moved in with my father and his shrew of a wife, and still the bad grades. I was making C's so they could'nt brag about me at all. They were very much the braggarts.They told me I couldn't work if I brought home anything lower than a "C" so i brought home straight "C's" until I moved out of thier house. I lived on my own on my moms social security money ($500.00) and a 15 hour a week job in a fast food restaurant, and surprise, surprise, straight A's that last semester. I had very good grades through college too!
I was in a particularly bad situation, and I am not saying your DS is going through anything like that, but maybe a third party could help him figure out the problem.
Also- I would see about maybe arranging with his teachers for you to go to school early with him and go to his teachers and drop off the assignments. Good luck!