It sounds to me like he's chewing the door for attention, or he's bored.
If the part he's chewing on is metal, it can actually be very damaging to his teeth.
How big is the cage, and what toys does he have? How much play time is he allowed outside of the cage? You can try giving him diffrent types of toys to keep him from getting bored. Diffrent types of chew toys, felt blankets, balls, cardboard, willow balls - rabbit toys don't have to be expensive. I build my rabbit little houses out of cardboard and he loves to chew on them and move them around his cage. I give him felt blankets to sleep on and he'll spend hours digging under the blanket and making tunnels under it. Rotate his toys every couple of days so they don't get boring to him.
Also, ignore him when he chews the cage. If he's doing it for attention, and you give it to him, even if your just going over to him to get him to stop doing it, then he gets what he wanted (your attention) and learns that chewing the cage gets him what he wants. Try to make him sit without chewing before you open the door to let him out to play. Ignore him if he chews, but if he doesn't go over to him and give him a treat or some pets. Teach him that chewing the bars isn't going to get him what he wants.
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