Well the first thing you do is sit back and relax, your son is an adult now, which means it is no longer your responsibility to arrange his affairs.
If he's being released to a half way house, this is far better for him than remaining in jail. If he doesn't want to go along with the directions of his probation officer, he can refuse and get comfortable where he is.
If he's already on adult probation for an offense, then adult probation already has his file and he will be instructed to report to them when he is released. In fact, the judge may have included this in the terms of his probation.
When someone is sentenced, they are usually given a number of general conditions they must abide by when they are released from jail on probation. This will include reporting to probation within one or two days of release from custody as well as following all reasonable directives of the probation officer.
So if the probation officer is requesting that he live in a half way house upon release, he will either follow that directive or find himself in violation of his probation.
Since it is apparent that in the past, he has continued to get himself in trouble whenever he was released and sent home, the request to live in a half way house seems reasonable.
This is being requested because the PO is trying to help your son get straightened out and stay out of trouble. By living in a half way house he will have some much needed guidance, counseling and supervision.
Be glad they are trying to help him get his life in order, rather than worrying if this requirement was ordered by a judge or not. Your son is now an adult and if he continues down the same law breaking roads he has traveled in his juvenile years, he is going to be spending a lot of time in adult prison.
The time for him to grow up and start obeying the law is now!