The consequences of not falling in line dont seem to be laid out crystal clear. He might think you not supporting him means youl let him live with you but wont give him cash. I dont know your son but hel probably have proverbial ear muffs on while he reads how much hes hurt you because addicts dont want to hear it. What stands out is what it means to him and his addiction and if you really are going to cut him off, I think you should specify what that means.
But i also see cynical one's point. He's learned that he can go to treatment, slack off, not DO anything, and instead of facing his consequences, he gets a letter and another shot. How many letters before its decided that more warnings arent going to do anything?
Not saying dont send it or listen to the counselor, just throwing it out there.
Hugs.