It could be going to fast. Too many classes can cause undue stress, and cause grade performance to drop. He's only in middle school, and here's one thing to think about- what's the worst thing that will happen if he fails? It's not a good thing, but good can come out of it.

Maybe if he fails this year, you can convince the school to evaluate him, or get him evaluated yourself. if you come to the school with an official diagnosis, they can't tell you no.

Maybe tell him it's okay to fail? As long as he's trying hard, the end grade doesn't matter, its the effort.

oh and the ' if you don't go to school, by law, i go to jail' is really effective if he starts up. my mom uses that as her counter argument and it works.