What a shocking experience that must have been for you.

Often a psych unit will bar visitors, even family, for several weeks. It happened with my son.
The medical and psych staff doing the evaluating don't know what the family situation is. Some family situations are violent and abusive. Keeping the patient safe means having to disallow visitation for a while.

He's going to be okay You're going to be okay

Right now, his evaluation is important so they can see what help he needs for the long term so he can have a healthy and meaningful and full life. This part will pass and then he will be on his way to feeling much better.