Also, how do I let him know this is bad? For when I find it hours after he’s done it and when I catch him in the act.
Another trick: Take your dog out for a ten minute walk four times a day: first thing when you get up, last thing before you go to bed, twice in between. If you do that, he may do all of his business outside. If ten minutes isn’t long enough, walk until he does his business—all of it. You know how often he poops in a day so obviously you won’t walk for hours if he just pooped earlier in the day. Once he knows he gets this treat of outdoor walking, he may never want to do poop indoors anymore and the two of you may be able to settle into a routine. Out first thing when you get up, last thing before bed, and a nice long walk of an hour or two when you get home from school or work. Hopefully by then the bathroom trips will only take a few minutes.
For professional dog-trainer advice, look up Cesar Milan’s website. He may have posted something about house breaking dogs.