How to potty train 4 y/o dog from puppy mill?

[Nebraska]

We have a 4 y/o Chihuahua/Yorkie mix that was rescued from a puppy mill in May. He's so small (3 lbs.) that he's easy to watch to make sure he doesn't mess in the house but once he's on his own, he goes wherever he wants. He'll mess in his crate so we tried leaving him in the kitchen and he'll do the same. I think he's been in a cage so long, that he doesn't care if he's lying in his own mess. Any suggestions?