Use strict supervision when he's out of the crate so you can catch him in the act if he does happen to have an accident. Interrupt him with a loud noise and quickly take him outside to his potty area to go. The key is to watch him constantly and if you can't watch him, crate him so he doesn't have the opportunity to go have an accident every time you're not close by. Just be sure to catch everything, because if he is able to get away with it too many times, it will become a harder habit to break.

When you do start leaving him unsupervised, do it for very short intervals and gradually increase the time he spends alone.

Good luck