Thank you for the replies.

There seems to be nothing wrong with him, but if I'm not mistaken, he was just in to see the vet 3-4 weeks ago.

He mostly wanders inside and outside, so he had no "real" reason to learn, so I'm wondering how I can teach him. Or, what to do with him because he cannot poop in the litterbox.

We have 3 litterboxes, and only two of them were being used by the 3 cats. They seemed content with that, and knowing that I'm rehoming some of them in a few days, I'm going to try to see if 3 will be enough.

Jack just doesn't know what to DO with the litterbox. How do I teach him? I taught my rats to use a litterbox, how do I teach a cat to use it when he's not in a confined space smelling his poop in the box? :/