It IS hard to stick to boundaries when we first set them. I recently had to make the decision to hold a boundary with my brother. It was really hard for me, made me feel crappy for a couple of days until I talked to my Sponsor and made peace with it. I know it was the healthy right thing to do. Maybe with your son, if you're not there to make prison any easier for him, he'll start to take a hard look at his choices.
You made the right decision. Hang in there.