I think that DMD boys lack skeletal muscle but they also lack the smaller dystrophin iso form in the brain. I am not saying that Sam is not smart - I think he is. But, perhaps b/c of the lack of dystophin, he learns differently and responds differently - at least that is what I have learned. I read that DMD boys can be easily distracted, respond more slowly and have certain areas that they have trouble with - i.e. phoenetic packages, short term memory, following multi step directions. I think you need to talk to his teacher and perhaps you could to go and see Dr. Poysky at Baylor University. He could do some acessments on Sam and see where he needs help and most likely make recommendations that would help him. Not sure if that would be a one on one aid but he is good b/c besides being a neuropsychologist, he has a DMD son. Please note that this is regardless of IQ...it's more of a learning issue and I think could be addressed with help and hard work.
Good luck Laurie and know that I am pulling for you and Sam!