Sorry to hear about that. Injuries are always very tough emotionally as well as physically.
I would think it would have a huge impact on him as a pitcher.
If he can't land on a firm plant leg (his left leg) because he's concerned about the injury or pain or whatever, I would think it would really be a hindrance.
That sounds like quite a serious injury and I would be more concerned about getting that taken care of before worrying about pitching.
When I tore mine many years ago (did the other one about 10 years later
) I couldn't play ball for a good 6 months, and then wore the brace while playing for the next 6 months). It wasn't a fun time.
But he's young and if you get a good doctor to fix it, and a good rehab program, he should be able to make it back 100% if he works hard.
He's only a freshman so you have plenty of time to do it right.
Good luck.