Make sure to close the door to any unoccupied rooms in the house (Pups love to mark in empty rooms. It's like the doggie equivalent of the "I was here" grafitti.) Try and make sure he goes out on a consistent schedule, as in, at the exact same times every day, give or take a couple of minutes. Clean whatever spot he's peed on thoroughly, so he can't smell his old urine -- bleach, steam clean, if on the carpet. As Skyrim mentioned -- watch him like a hawk, while he's in the house. Pups show signs before they potty -- it varies from puppy to puppy what those are. If all else fails, tether him to yourself, as you go about your normal business around the house (also a good introduction to loose-leash walking.) If he starts squirming, let him outside and praise him generously/treat when he pees where he's supposed to.
Della - GSD- DOB 10/23/13
Dina - GSD - RIP 09/01/98