While I don't know him or your step daughter, only the situation...I'd keep my eyes and ears open. You know his past and what he's capable of...they are both children, raising this baby.

Maybe this will be the "thing"* that makes him grow up, maybe it wont. But if its making him get a job and seemingly act like a more responsible person, he deserves credit and praise for it. I think anyone trying to straighten up needs encouragment. But always, always listen to your gut. Its nice and good to give chances and keep an open mind, but keep an open eye/ear too.

*thing...not a thing, a precious little boy, but you know what I meant.