Is he by any chance a Manx? I was just looking into the Manx cats, seem they get "poo-dangles" frequently. Something I deal with alot with our young girl Heidi. It has something to do with the lack of a tail and the tail somehow being a way of knocking the poo-dangle free when a cat lowers it's tail upon completion of a trip to the box. If he's not a tailless kitty, I have no clue what would be causing it.