When's he going to treatment? That's good news he's agreed. Of course... the follow through is where addicts usually screw up. The road to hell is paved with good intentions and all that jazz.
How do you know he's clean right now? Did you drug test him or something? Being clean is not the same thing as recovery. It's usually just a relapse waiting to happen. Or a lie.
RE: the debit card. At 24 I used to give my mom money to hold onto for me - to put into her savings account so that I wouldn't have access to it. I needed her help to save money for a trip. I followed through on my end the bargain, so she followed through on hers.
It's up to you mom. Give it chance and if it doesn't work then give it back to him and tell him he's on his own - totally on his own. Youll know soon enough whether he's committed to staying clean.
In the meantime, are you doing anything for your own recovery, so that, if necessary, you'll be strong enough to follow through on the boundaries you have drawn? How are you going to distance yourself from his problem and let your little boy grow up to be a man and face the world on his own?