Behavior for puppy 8 months

I have a 8 month old pitbull boxer mix that I have been fostering for over a month.He had kennel cough when we got him and is all cleared up other than that he’s been a perfect dog except…he goes to the bathroom in the house.He goes an excessive amount both poop and pee when he goes out 6x a day.I’m not joking I’m stressed two of the six times he goes out is for an hour and every time he’s outside he poops and pee.I don’t understand almost every time I’m not in the same room or turned around he does it.I know people would say kennel him I was thinking it could be separation anxiety but when I tell you he goes whenever I am turned around or leave the room for a second I mean it.I could go pee real quick and I’ll walk in the living room and they’ll be a big pile of poop and a puddle of pee.I could walk in the kitchen and walk back out and it’s covered in poop and pee.I’m super stressed because I already take him out a lot and kenneling him every time I walk to the kitchen or turn around is super annoying.What could cause these accidents?I’ve only caught him in the act once I have taken him to the vet twice and other than a cleared up kennel cough he was fine.