Layla's vet said if she starts to break her housetraining it could be a sign of a bladder infection... i think they just do blood work. Poor guy. I would bring him to the vet so at least they can rule that out and you know it is marking. I'd hate to know i was getting mad at him when it's not his fault... that look they give you... oh can break hearts
If it is marking... you can always get a belly band.