It is very likely this kid could have very easily subluxed his shoulder before causing the tear. Once that happens, it doesn't take much for the humerus to come out again and again.
Unfortunately it sounds like surgery is most definitely needed and the outcome is very much in the air. Surgery will fix the problem, but it won't solve the issue. There has to be some sort of underlying cause..
Also, post surgery it will be very important that rehab takes place and that he follows everything to a T.
Hope for the best and prepare for the worst... I know it stinks, but remember the inability to play the game of baseball is not the end of the world...