It depends. Pictures will help! Bettas, like most freshwater fish, have the amazing ability to grow back missing fins. Small tears heal very well pretty quickly, and even enormous tears and entirely missing fins can heal nicely. Just keep his home clean! Just out of curiosity, what size tank is the lil guy in? :)
Might want to put some aquarium salt in the water and keep his water clean, maybe even do more water changes then you usually would to prevent infections. Another thing that will help is to make sure his water temp is stable.
Like Bettaluver4evr said, they're pretty amazing little creatures and most of the time their fins will grow back.