Frankie's Latest Trick

The dog's latest trick is not so much of a skill as it is an annoying relapse into puppyhood. Somehow he's decided he's had it with going to the bathroom outside, and will go in the house whenever the mood strikes. Especially right after having been walked. Clearly there are issues going on in that grapefruit sized brain of his but whatever is causing it, it means we have to go through remedial housebreaking training. Joy. Just what I wanted to do at 34wks pregnant in freezing weather. Anyone have any tips on getting him to poop on command? Or reasons why he doesn't associate being on the leash with going to the bathroom?