Luckily I've never had to deal with a memory loss situation like this with a friend but like the other brotha said above me just be there for him, and be there for his family. Help them out however you can. I'm not sure how old you are but you or your mom/dad should make them a meal you could bring to the hospital if they're staying there in the evenings. When I was in the hospital a friend did that for my parents and me it was nice to have home cooked food rather than the hospital stuff and my parents appreciated it because they had been going out to eat rather than cooking obviously.
But yeah man just be there for your buddy vibes to you, vibes to his family and major major vibes to him. May the sun be on his face and the wind at his back, and I'll keep him in my thoughts hoping for a speedy recovery.