A friend went through this with a soccer concussion, she spent most of the fall semester on homebound school. She was able to graduate with her class. She chose to stay home at community college her first year, because of reoccuring issues. Smaller classes, fewer courses in the fall and did well. In the spring she took 15 hours and then started at a university this year.
There are lots of online options for when he is ready, but there should be a homebound option also. You may have to get the school social worker involved.
You cannot control his recovery, you cannot control his stress, but you can assure him that if he follows the doctors orders he has a great future in front of him.