Do you actually see him going on the stairs? Also, I'm not sure if you're talking about pee or poop or both. Sadie is a very private pooper. She likes to go on a pad that is in a room away from us. I thought that she wasn't using the pad because the poop was never on the pad. One time I was able to see her "go" on the pad - only nothing was on the pad. As she trotted away the poop fell off - halfway across the room. Is it possible that he actually thought he was going on the pad but it didn't fall off till he was on the stairs? Of course if you're talking about pee - none of this applies.
