Can you post a picture of his tank? Because sometimes what happens to Bettas is that their tails get scratched by decor. Also, some Betta's fins don't grow as fast as others, my Betta Angel's tail was bitten off in places back in 2011
and is almost done with regrowth.......
What can help to get the growing going is doing your regular water changes (gallon 2-3 times a week 4-5 gallon twice a week and five and up once a week) and you can use some aquarium salt to help heal his tail but use as directed.
And for fun, to see if he is tail biting this might require a little work but I find this fun (I used it as a anti cat surveillance) set up a web cam and use two different devices, one you can watch at any time and one focused on the tank. I hope you find this info useful! Good luck with Winchester!