[FirstYorkie]

You shouldn't punish him. There is absolutely nothing you can do if you find an accident later but sigh and clean it up. If you catch him in the act, you can give a loud "NO!" to interrupt him, scoop him up, and take him outside to finish. Praise and reward him if he does finish outside.

Be sure that he is being adequately rewarded for pottying appropriately.

When you can't watch him like a hawk, keep him contained in an area where he is less likely to have accidents.

Good luck!