My Son Fell Om His Back And Hit The Back

My son fell om his back and hit the back of his head hard five years ago.. Which resulted in a huge bump on his head which he ended up having a concussion.. After he healed the spot where the bump was his hair never grew back so therefore he has a permanent bald spot on the back of his head! What can I do to treat this ?or is there any procedure I could do for him to fix this problem?
Thank you!