Hello Kitty,
The key to getting back that mobility with the arm is starting PT asap! And as soon as he can handle it, massage the neck tissue to desensitize the nerves and soften the tissue. I have some weakness with my arm after all these years but it does go up! Other PT issues to think about is opening the mouth as wide as possible many times a day. I didn't know that one when I was in treatment. With dry mouth issues after treatment he will want to be able to open wide so the dentist can get back there and do his/her thing. And being able to eat a hamburger would be nice.
Best of luck to you!