The best way that I can think of is to acknowledge that the responsibility is really on him. And then ask him "how may I help you keep yourself out of prison?" or "what are your plans?" and then decide if and where you would like to be supportive in his plans and make that offer. He needs to be in the lead on this. If he is interested in turning his life around, he'll tell you and if he isn't, then there may be not much you can do.
While taking addiction programs into the local prison, I met some people there who were really turning their lives around. They introduced me to a book called
"We're All Doing Time" by Bo Lozoff, an excellent practical guidebook that you both might want to read.