Just want to welcome you to Yorkietalk, I've had a few older dogs that weren't house broken, but they weren't puppymill dogs, so I'm not sure if my suggestion will help. I would just take them for a walk every few hours, and praise them when they did their business. There were accidents, but not many. Dogs seem to really like going outside, even Joey who is pee pad trained, seems to prefer going outside, if it's not too cold or hot. Bless you for taking this little guy, and can't wait to see pictures. I found some great tips on this website.
Free House Training for Puppies and Adult Dogs - No More Accidents Starting Today!