You could ask your vet if they have a product called NOSORB, it is little tiny pebble like things. They come in a sterile cup and you pour it into a clean litterbox with no litter, just the NOSORB and then separate Jack so he is the only one with access to it. And then when he does finally pee, pour the NOSORB and the pee back into the cup it came in and bring it back to the vet. I think it's under 10.00.

p.s. I just noticed that you are a vet assistant so you're probably laughing right now since you know all about NOSORB! LOL