Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine your son and obtain a history, I can not tell you what the exact cause of his symptoms is. However I will try to provide you with some useful information.
It sounds like your son has residual symptoms of weakness and numbness after a sports injury. If he has had a normal MRI of the brain and cervical spine, this implies that there is not damage large enough to be shown on the MRI. This rules out extensive damage, bleeds. However, if the impact on his head was strong enough, mild injury due to stretching of the nerves in the brain (diffuse axonal injury) or something similar to a bruise (mild contusion) could potentially explain his symptoms. In such cases, physical therapy is important, as is counseling if he has sufferred significant stress related to the ordeal. Time is required for improvement in such cases, and persistence with physical therapy is important, as months are sometimes required for full recovery. I recommend continued follow-up with your neurologist, paritcularly if your son's symptoms do not improve or he develops new symptoms.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.
I would go to a CranioSacral therapist over a physical therapist for issues like this. I am a bit jaded about the physical therapy profession, but I find in general their knowledge base is not up to date. This is not to say there are not a few good PT's out there that have studied beyond what they were taught in school, just that most of them have sub-par educations based on outdated models.