Can you post a picture?
What size of tank, what is the temperature, how often are the water changes, and do you add tap water conditioner? :) Many questions! But it'll help
Have you tried toys? Like a ping pong ball? Or avoided cupping him during water changes? Try rearranging decor... Or even having him in a higher or lower traffic area!
Keep in mind these are ideas to help or avoid tail biting, but some are just chronic, and sometimes for the reason of their fins being too heavy. Take a look at this fella: He chews his fins now, because his fins are always growing and he was being weighed down!!!