One thought: If I correctly recall, one of the pieces of advice which are put forward at the start of discussions on the subject is that drug taking quickly shows up as "out of character" behaviour, sharp mood swings and general unpredictability. Have you considered the possibility that drugs may be involved ? I agree with pretty much everything offered on here up to now and not least the two points that you should not take your son in without your husband's presence and agreement, and that your son must take steps to get help. Unfortunately, there have been too many cases of the system being very slow to react and even turning those in need away.
I think you can sense that all responding to your post want to be of some help and most/all (certainly those with children of their own) probably have a gut wrenching sense of "but for the grace...".