Welcome to the site. Thanks so much for rescuing the poor little guy. We have a couple of bullies here that don't squat when they poop, such as @Libra926's Vegas. How about peeing, is he able to control that? At 4 months and not being previously trained, it's very understandable that he doesn't know the proper peeing/pooping etiquette. It's going to take some time, but for now, I would set a schedule for him to do his business outside, like puppies. For instance, I'd try taking him out every couple of hours, and every time after rough play, right after he wakes up, right before he sleeps, and a couple of times in the middle of the morning.