What is the saying? Remain cautiously optimistic?
You are doing the right thing, standing by your insistence that he not come home. Tell him that his living there was a privilege that he lost, but can eventually earn back if he so chooses.
It is not unheard of for someone to see the light when they are in a less than savory situation. I was not in prison, but when I was in a DV shelter almost 4 years ago, that is when I got clean this last time. And it was one of those fall on my knees, pray to God, watch the heavens split open above moments. Seriously, this was a major "spiritual awakening". Of course I was 34, not 17. Not that it can't happen...I'm just saying...
Be safe. You have come SO far. Stand back, and let him do his thing. Do not patronize him if he does not stick to what he said he would do.
He will be in my prayers that he does, however.