I would go back to basic house training. Most dogs have one or two regular BMs a day, usualy 15 - 30 mins after eating. After every meal, take him outside to his spot for a few minutes only. If he goes, lots of praise and a short (or long!) walk or play time immediately. If he doesn't, take him inside and put him in his crate (with a toy) for 15 mintues -
then take him out again for another chance. No potty, back in crate. Keep this up till he goes, then praise and reward - food, play or walk. This may take a looong time te first few times. Eventually he will learn that the faster he goes, the faster he gets freedom and gets to play.