For healing just keep his water extra clean, do extra water changes if you can. Add some API Stresscoat and a possibly a pinch of AQ salt to prevent infection.
As for preventing it, that is harder to do. Bettas tail bite for all sorts of reasons.
Because they are bored - so you can try rearranging the tank
because their tails are just really heavy
because they are stressed - add more plants
because they do not have an adequate diet - try feeding him pellets that are low in fillers like Omega One or New Life Spectrum. Also try feed him some frozen (not freeze dried) blood worms.