Welcome to Pet Talk! I also have nothing to help you, but I wanted to welcome you nonetheless. You are a womnderful soul to take in such a needy kitty.

Where did he go when he wasn't using the box? The reason I ask is because my Allen sometimes does and sometimes doesn't use the box. Always in the same place - the bathmat right in front of the litter box. He for some reason likes to poop on the bathmats, even if his litter box is clean. I've just come to accept this as an odd quirk of his.

Oh, and does he use the box for pee, then not poo? Some cats are fussy about that... the box is otherwise clean, but they won;t use it.