I know some people disagree with this, but this has REALLY worked for me with both of my yorkies both whom I trained to potty outside. When you see him going potty inside, pop him on the but with an empty paper towel roll, tell him NO, and take him outside. Then, if he finishes up outside (or whenever he goes potty outside at all) have a treat with you to give him right then. Then praise, praise, praise. Be VERY consistent. He is still really young so DON'T give up!
It sounds like you are doing well so far. I promise you that he won't think you are playing if you pop him on the butt with the paper towel roll. This DOES NOT hurt them at all...it just makes a loud sound and scares them a little. I even pop my two with it if they are chewing on something they aren't supposed to (then give them the proper chew toy) or do anything else bad. Now all I have to do is show them the paper towel roll and they know they've acted up.
I will be the first one to admit that some people probably disagree with this method. But, it does not hurt the dogs and it is something that has worked wonders for us.