| Two things I would do/did:
1) Take him out more often and lots and lots of praise when he goes outside.
2) Keep him in sight indoors at all times so keep the doors closed to bedrooms if your in the living room, etc.....when he starts or it looks like he's about to go, scold him, yell, clab your hands, startle him somehow so he will stop, go over pick him up, carry him outside, and let him finish and then lots of praise/reward again.
If your consistant, he'll figure it out! good luck |