You should talk to an attorney about suing everyone involved.  The hospital won't comment, because they're afraid, rightfully so, that anything they tell you might come back to haunt them in court;  if I were the hospital's lawyer, I'd have told them that talking to you would mean their jobs.

You're likely to have some problems with this lawsuit, because a bipolar patient tends not to be the most credible witness about what happened while he was in crisis.  Take pictures of his visible injuries now, all of them, multiple views, if you haven't already.  Sometimes, those pictures can prove critical facts, such as (in this one) whether he was beaten after he was already in handcuffs.  There's simply no legal justification for that whatsoever.

I would like to add more input on this question.  You should contact an attorney immediately to investigate this type of case.  There may have been witnesses to this incident who could help corroborate what your son has told you.  The longer you wait the more likely it is that valuable information could be lost which might be developed through an immediate investigation. It is a very difficult situation and I understand as a parent how painful it must be for you to see your son go through so much heartache.