When all the people have advised rest, they meant it. You've been on high alert for so long, that your entire body needs replenishing - sleep, relaxation, a long exhale of air from your lungs. Because after you've had a little rest, you're going to learn what you can about your stepson's issues, you're going to advocate for him to get him into the right treatment programs, you're going to deal with school issues, doctors, insurance people, and probably law enforcement and the judicial system. Some of these people can be as difficult as your difficult child.
Your stepson needs to know his father loves him, and wants what is best for him. There will be many opportunities for your husband and his son to have contact even while your step son is away.
If your stepson is allowed to have a phone call (normally a few times a week for 5-10 minutes), as long as your stepson is behaving reasonably well on the call, your husband needs to take (or make) the call.
Most places allow their residents to receive mail. Send cards, letters and pictures. He may not be able to keep everything he receives in his room, but he will know the frequency with which they are sent.
When visits are allowed - if at all possible, go.
Give husband a little time to blow off some steam - as I am sure there is plenty he needs to let out, but guide him back to doing his job as a father. Your own son is watching husband now, too. Does your son get to see a grown man try to move mountains when the going gets tough, or does he see the grown man throw in the towel?
I concur with the group - family counselling is needed for all. This is a long journey you're all going to be on.
So glad you found us, and you're finding support here.