First is the mri to see basic damage but keep in mind you will not know full damage until the surgery is actually taking place. Next is to talk to at least two surgeons and as some have already suggested I would contact surgeons that work exclusively on athletes, check with your pro teams in the area. The surgeon will also have the rehab people set up for you. Very important.
The rehab starts very quickly and is intense and tough, your son will find out quickly just how important baseball is to him and if it is he should come out stronger than ever.
Couple other thoughts, one of the keys is no infection so they don't have complications, ask Tom Bradey. They also have a machine that wraps around the knee area after the surgery that circulates ice cold water around the area which really helps with swelling and early recovery we found very useful.
last but not least, make sure you and your family are there for solid support thoughout the recovery to keep everything positive. The mental aspect can be started right away so he does not favor the injured leg and do damage else where. Sounds easy but like most things sometimes the mental is as tough if not tougher than the physical.
He can fully recover from this and of course best of luck to your son.