Boredom, heavy tail, stress and overly aggression can cause tail biting- it's like us biting our finger nails. It's a bad habit they pick up for one reason or another and to stop it is very hard. Halfmoons like yours are very notorious for biting their own fins.
Just do regular water changes accordingly to your tank each week to prevent rot and he should be fine.
Some ways to try to stop him have been mentioned by Bom :)
Here is a thread
about some of the member's tail biters you can check out, share the pain along with the rest of us with tail biters!
Captain: Biting you will see more of small C or U markings at the edges of the fins, sometimes a small V if they pull rather then use their teeth to bite through.
With rot you will see blackened, flaking, crusty type edges, or white puffs of cotton like substance.
Off topic: Rj, what type of plants are those in his tank? Some look to be bamboo.. want to make sure since bamboo shouldn't be in the tank with them.