I had a betta I nicknamed "Wiggles" because he loved squeezing into small spaces. To the point of danger to his life. He once vanished from my tank for a couple of days, I moved everything around, pulled out the plants, and he just wasn't in there! Fish don't just vanish from their tanks, so as I pulled out his cave in puzzlement I discovered he had somehow wiggled his way into the hollowness of the ornament and was INSIDE of it, trapped. When I put it back in, I buried the bottom of it in the gravel flat against the bottom. Again he vanished, but I found a spot he had dug out so he could get back in! I bought him a new cave ornament, with no hollow interior areas he could get stuck in. After that he began actually pulling the rocks out from under ornaments so he could squeeze under them...He was harder to find than ghost shrimp and he was bright blue!
I'd suggest getting him some small ornaments he can chill in, some fish just like small spaces!