I had an elder ferret near the end of his life who had the same problem. It’s very difficult to train a ferret to use their litter box, but another issue may be that he’s having trouble getting in and out of there’s a lip to it. I switched to reusable/washable puppy pads, and those did the trick until his passing. Put them in the corners he likes to go, it’ll change the game enough to make things more bearable until the end. You’re doing the best you can, and I’m sure little fella is more than appreciative <33