You could always try just flattened cardboard boxes, that way he can destroy them, and you won’t have to worry about it harming him at all.

I’ve noticed that my bunnies will destroy anything if they can get a hold of the edges of it, so I purchase cheap rugs from discount stores (like Ross or Marshall’s) – that are larger than the cage. That way, the edges are on the outside of the cage. Recently I bought those bamboo mats and those have worked well because my bunnies can’t really pull on them, and the edges are outside of the cage as well.

Cardboard boxes like Monkeybun suggested are also a good idea – that way his flooring will also keep him entertained! I also usually put a tile square somewhere in the cage for my bunnies to lay on in case they get too warm.

I really like large seagrass mats. They are flat, roll up easily to store them, and are very good for bunny if he chews on it. They’re not expensive, so your could even put down some and then others on top so he doesn’t chew the bottom layer.

Once you have your flooring sorted out, provide him with something to chew that is safe. A simple thing is a cardboard box. I have some around the house that have something heavy in them and my bunnies go to town on them. I find if the box stays still, they chew at it for ages = cheap entertainment. Even just an empty one they can sit in an tear at the sides. If you do add something like that to his pen, just ensure he can’t launch off it and over the pen side!