Ok, I'm not saying I KNOW what your son's motives are, but I've been down this road at least 10x with my 22 yo AS. I am interpreting your son's "plan" as a way to get you hooked into his welfare. "The Plan" is one he knows you're on board with so he can definetely use that to get some leverage and gain your sympathies for whatever situation in which he finds himself in the next few months. Subs can be sold on the street for his DOC, of course. They do it all the time.
He's 26 and has a job. This would be an excellent point for you to memorize and use the phrase, "That must be hard for you. I'm sure you will figure something out."
And, by the way, you are allowed to change your mind about paying for rehab/detox and spend the money on a nice vacation for yourself instead.
Just my $0.02.