Welcome to SR,
You have to let him hit his rock bottom. If that means he losses everything then he loses everything.
I know it is hard because I have been did the same thing for years. My Ex is on his third rehab stay and I have been there for him every time. But what is different this time is me.
Set boundries and keep them. Do not pay him food, if he has the money for the pot then he has the money for food. He can make that choice of either buying pot or food.
As for you keep close to his family you can do that without him in the picture. Have you talked to his family about the pot smoking.
Your son needs you and no body else. Trust me when I say your son in the long run will be fine. He might ask where he is at the beginning but you have to tell him it had nothing to do with him that you and the ex just need to be apart.
If he really loves himself then he can love you. YOu can not fix him, you need to go to meetings yourself.