I have a tail biter too
My advice, and what helped Dragonfly, was upgrading him to a larger tank. I ended up getting a 15 gallon and dividing it
Heater and sponge filter, which he LOVES swimming in and out of the bubbles! I also make sure the water is a nice 80F, seems to help. I have TONS of decorations in the tank, as well as some I keep outside the tank! Once a month I switch around the decorations, remove some and add new ones
I also find giving lots of different "caves" to explore really help. He also LOVE playing in his Betta Log.
Hope this helps!