Perhaps try and change his toys around so it doesn’t get boring for him ? So have fewer out at once, but change them round once or twice a day to keep him interest. In my experience though very intelligent bunnies have quite short attention spans! My bun Pickles really likes having cardboard of any kind that he can throw! or you can stuff little wooden or paper boxes with hay or Rookie’s favourite treats and then he’ll be too distracted.
Should keep him entertained for hours.
Also, if that doesn’t work, I’ve heard of people putting citrus juice on the bars to stop the gnawing, but I don’t know which kinds are okay for bunnies….I wouldn’t recommend you try this without doing some research as to what kinds of juices are okay for them but bitter enough to stop the gnawing. I’ve never had this issue with a rabbit before but have used lemon juice to calm down a hamster with cage rage/ bar-biting issues.
Also, is he in the same room all the time ? That can get boring for bun. If you have any other rooms that are safe, try letting him out into those for a while (under supervision at first)…..Pickles I’ve found likes to come into the living room with me and watch television, and a lot of our ‘play sessions’ start with a selfie shoot on my bed which he always thinks is great xD
Sorry for the essay…but hope this helps!