2014Parent,
Yes, I think your son should call the coaches when he is able. If he is not able to respond before any imposed deadlines, I think that is different situation. Then you (parent) makes the call explaining the situation.
I recall having a similar offline discussion with another poster last year about their son's injury during the recruiting process. leftyshortstop reminded me of that dialogue with his words and phrases.
First of all, I hope your son has a quick physical and mental recovery. Getting back on the baseball field may be the furthest thing from your mind right now. Everybody on this message board can empathize with your situation. Where I'm going with this is that I'd try to use this situation as a filter to find out who (coaches) is truly interested in your son. If they were truly interested in your son's well being and health there should be something beyond some surface questions and dialogue. As parent's, our first instinct is to protect our kids and get them healthy asap. Your son's situation is fairly unique at this point on the recruiting calendar, and I'd like to suggest it is a way for him and you to find out who is really interested in him as a human being. The feedback I received from last years offline discussion with another poster was exactly that. It can be a way to separate those very interested programs from casually interested. JMO.