The fact that he's an appy isn't going to make growing out his tail any easier on you... how do you know that he's not rubbing it out? it wouldn't hurt to check for any skin conditions, even dryness can be irritating. If he is rubbing it out, the tail would look spars. if it's another horse eating it, it will look choppy. maybe if you posted a picture...? there are commercial products you can buy, which supposedly taste terrible (i wouldn't know :P), therefore stopping the chewing, but from what i know, chewing is a nasty habit that is really hard to break.
Good luck, i've never had to deal with this personally before (knock on wood) ;)