If he has chew toys he could be bored, stressed, or frustrated. I'm not sure how you house him or how much exercise & interaction he gets but if you haven't, maybe consider bunny proofing your home and letting him out of the cage more. Or let him outside in a playpen, just keep him occupied. rabbits can be occupied by digging, eating, exploring, and social interaction. They are naturally most active in the evenings and early mornings, and sleep most in the afternoon. He is probably going to chew the bars when confined at a time he is most active. Try noting when he sleeps and when he is active and adjusting your routine so he gets exercise during the active period to wear him out.