Bettas definitely love to have some cover. Especially if they're stressed.
Definitely put some silk plants in with him if you haven't already.
Each of my bettas has their own tube/cave/pot as well as several plants in each tank.
One has a small ceramic flowerpot (with a frog figurine straddling it), and the rest of my guys have those clear hamster tubes. I really like how those hamster tubes look. It's definitely not "natural" looking in a tank but the clear color ones add such a pretty splash of color peeking out from behind some plants. Plus the t-tube shape is great. I place it so one of the openings is pointing up - easier for the betta to find his way out. All my guys love their tubes. Most of them sit in it with their heads sticking out, just like a dog in his doghouse.
I'm a bit paranoid ever since I saved one guy when he swam up inside a decoration and got stuck. Thank god I was paying a lot of attention to him that day and panicked when I couldn't see him. I lifted up the tree stump decoration and he swam out from way up in it. There was only a tiny pocket in the gravel where he squeezed himself under. I doubt he would've found his way out.
Funny thing, this guy was a crowntail too. I wonder if they have a burrowing trait? I've never seen any do that either.
That's an important thing to remember when looking for decorations for bettas, make sure it's not one of those hollow things that the betta can get up inside and not find his way out for air. If so, you'd want to fill it up with something.
A lot of those castles wouldn't be good because there's a big pocket up inside that an air breathing fish could easily get stuck and suffocate.