If it was me I would be filling the space up with tunnels/hides - you don't have to buy things, you can reuse cardboard boxes or tubes. Anything that he can run through, over and tunnel under. Make it like a mini obstacle course, then you could scatter feed in amongst it, if you don't already and chuck in a few toilet roll boredom breakers every now and then.
in case you don't know what I mean - something like this
https://www.youtube.com/watch?v=qQNJ...2&index=3&t=2s
I also can't see where he sleeps, you could also use a shoe box for that, take the lid off, turn it upside down. so it has no floor and cut a door way into it.