Maybe I am in the minority, but when my son is discharged from residential in a week or 2, and goes to sober living/IOP, I will not be paying his rent. He is 23, and very capable of holding a 1/2 or 3/4 time job. We will help him with the 1st month only. Does your son have a job? If he does, in my opinion, he should be contributing to rent. If he has a job, and is not contributing, I'd be concerned with what he's spending his income on.
They need to made to get off their 'rear ends', for many reasons. Main one is to keep their minds busy, another big one, is that it helps to give them responsibility and pumps up their self-esteem (which most users have little of). I would help him with food, but that is it. Even that is tricky (you will read down below why). His phone is already being paid for by his paternal grandma. Why cannot he handle the rent?
A few months back when he was renting a room & had a job, he was adamant that I put a certain amount of 'food allowance' in his account weekly. I did, and you know what that was being used for? ALCOHOL and WEED. I found out when I realized I had his bank account password written down, and did some snooping! It was a total pattern. I'd transfer $ over---->next transaction? A stop at a local 'liquor store', an 'Ale House', a CVS drug store!
Also, he was making pretty good cash by delivering chinese food. Then, he'd be telling me he was short on rent and the man was letting him make it in pmts..I wondered, 'why on earth, is he not able to pay that small amt in rent with what he's telling me he makes each night?' A-ha! He was spending his income and tips on ALCOHOL and WEED.
I am not criticizing, but I feel strongly about this. Everybody does what 'they' need to do, on their detachment route. 
I like this, Copa. Chose, choose, choice, decide, decided, decision - good terms to keep in my arsenal for next week's 3 day Family Session, and the future. It puts it on them.
TL - my son does this all.the.time. Calls, is nicey nicey. We get off the phone, and I wait. Wait for the next call, which will inevitably be, asky, asky. Then, when I say 'NO' ----->it's nasty, nasty.
Soooo predictable.