He has multiple chew toys available? Try getting something made out of apple tree wood, it's really good for chewing rabbits from what I've heard.

When I had my rabbit, he would bite the metal bars and pull to make a long ding sound. He would also chew on his water bottle. I think he just wanted to chew on metal for some reason, because he had a ton of wood to chew on (which he did as well).
It could be just a habit. The only harm it would do is make noise.