Keep him tethered to you...and watch him like a hawk. When he gets into position, pick him up and take him outside. "Throw a party" when he does it outside. Use a specific word..."good potty" or "good poop" or whatever. Also, if he poops inside, you have to clean that spot completely. Use the special pet cleaners.
I got my puppy at 8 weeks old and he pooped in the house for the last time 3 days later. However my problem was the peeing. It took him months before he stopped peeing in the house. Also, my dog (now 19 months old tomorrow) is a creature of habit. It makes my life easier because I know where he likes to go. He tends to poop in the same spots. Then I don't need to walk forever. (I don't have a fenced in yard so I can't just let him out back.) If you try to keep a regular schedule, eventually he'll follow that schedule better than you can. His internal body clock can be quite accurate. For example, if I'm doing something at 5:30p, he'll come to me and harass me letting me know it's his dinner time...every day, without fail. So...structure is what dogs need for your sanity. You try to stay on the same schedule everyday and your dog will be satisfied and your life will less stressful....at the very least when he's still young.