Dear Despairing Dad~
It is a terrible thing to see your son going off the rails like that, he will probably seem nothing like the person you brought up, except maybe in short periods. If his mother or other family are involved it will be terrible hard for them too. Grieg, frustration, feelings of despair and even guilt can be almost overwhelming.
I would suggest that you -and any other family -seek counseling and medical support to keep yourselves as well a possible.
I'm sure you will have tried everything you possibly can. Talking does not good, even getting hm to see there is a problems can be just about impossible as times. I guess the one bright spot is his girlfriend who sounds very sensible, though the strain on her must be immense too.
I do not know the arrangements in your state, in mine there is a crisis assessment and treatment team (CATT) who can be called in immediately to see if a person has the ability to take care of themselves and if they are a serious potential threat of harm to themselves or others.
They can then place the person in a secure environment such as he has been in in before and after a hearing involving two doctors and family can be placed on a compulsory medication order and released. This order normally involves regular assessment and therapy as well
If the medication is not taken it is given involuntarily.
This is all very difficult and the crux of the matter has to be the findings of the team at the time they are called, without which the whole chain of processes does not even start.
I do not know if this whole system exists in your area, or if calling such facilities is feasible. I guess if he becomes in one of the states I've just described. Can you say if in fact such systems are there for you?
I know the above is only a slim chance, normally a person only received effective treatment if they realise there is a problem and wants to take an active part in the treatment.
So the only think I can say that I'm sure will do good is for you to look after yourself. While the problem exists you need daily respite, and another side to your life away from this. May I ask if you do have medical/counseling support? Also if you have another to help share the burden?