My rescue dog poops like a horse

I just rescued a GSD retriever mix several months ago. The foster family estimate him to be 1-2 yo. He’s a really good dog. But one quirk I noticed is that he doesn’t squat when he poops. We would be taking him on walks, then suddenly mid walk he would pause, tense up, million mile stares, and start pooping while he’s in his walking position, he would often times pees at the same time.

This is a minor inconvenience because we would often leave poop stains on the sidewalk since he would not go to the grass. We would like to train him if we can but I’m not seeing anything online. At this point if I can get him to poop in the grass or dirt that would be a huge improvement. Is it possible to even teach him to get into the squat position?