He needs occupation? Well, I can think of something that might keep him occupied. I've heard of a dog sitting in front of the television watching a dog film. And was somehow emotional about it. If I exaggerate slightly, the dog in front of the TV was sad and howled every time the dog on the screen did it.
So let's apply this experience to your situation, because why should it be any different for otters than it is for dogs? The solution: You, Magdalene, open Nightcafe and show your little friend some of the otter pictures you have created.
Then the otter surely will delight you with a smile - and will start painting. I am very interested to see what images he will create on paper. Best wishes from the Otter Advice Centre in Germany.