Please answer these questions on this thread and post them: http://www.bettafish.com/showthread.php?t=49233
OFL recommends 1 tsp Aquarium Salt per Gallon of water to help prevent infection in his fins with 100% water changes daily for a week. Do not use aquarium salt more than a week because it can cause strain on his organs. Just keep his water clean and make sure you don't have any fake plants that can snag and rip his fins even more. (Use a panty hose test..run the fake plant along the stockings and if it snags, it will probably rip your betta's fins). Many people recommend using live plants.
It really just depends on the fish how he heals and how high in protein his diet is...what are you feeding him?
He will also need a heater (I prefer my temperature at 79ºF, but many people suggest anywhere from 76-82º).