I think you're going to have to exercise "tough love", Jasper's Mom. And watch him like a hawk! Maybe you can do something inside to make him happier, like stacking some boxes, get him a huge scratching post he can climb, and make sure he has good perches near windows so that he can see what is going on outdoors, without actually letting him outside. And I don't know about Jasper, but my kitties love open windows, where they can lie on the ledge and smell and see everything outside.

I think the leash and harness idea is a good one, but I'd wait a while, and force him to get used to being inside. If he experiences the outdoors, he might decide he needs more of that.

Good luck. Give him lots of attention and distractions, and maybe in time, he'll get used to it.

Logan