Carmen is right, the chewing is a sign of agitation. You say it's a wire bottomed cage? Is there anywhere else in the cage, besides the litter tray where your rabbit can get his feet off the wire? The larger the breed (what kind of angora have you got? They generally weigh anywhere from 7-13 lbs, depending on the breed, with the exception of a few miniature breeds), the more irritating the wire bottom cage can be. Also, the litter tray should be as far away from the food as possible.
If there is enough room in the cage, try slipping a piece of drywall/jiprock in the cage for your bunny to rest his feet (it's safe, even if they chew at it). You could try a piece of wood, but wood can stay damp and cause even more damage to feet that are already sore from the wire.
Also, if you look on the bottoms of your rabbit's feet, do you notice any sores or scabs? Those are "sore hocks" which means that those sores are caused because of the wire bottom. Usually, providing something to rest on will cause those to clear up (again, wood can get damp and cause those sores to become infected). That may be why he sleeps in the litter tray all the time.
With a wire bottom cage, usually there are pull out trays underneath, so there usually isn't a need for a litter tray -- if I may ask, why did you decided to use a litter tray?
Most of the time, rabbits will pick one corner to go to the bathroom in most constantly, even though a few escaped poos will find themselves elsewhere (either as marking, or just because he couldn't get back to his corner in time). This corner is usually the opposite corner from where the food is.
How long have you had the rabbit for?