Clean water is a must! If you would like you can add about 3-5 drops of BettaFix to his water as well. I know alot of people don't like the stuff, but it's always worked for me, just don't over use it. You can add it daily but usually no more then 5 drops if he's in anything smaller then 2.5 gallons, if it's a 5+ gallon tank you can add 10 drops. I've never had a problem with it, it's always worked great for me.
Also feeding frozen or live foods can help with fin regrowth. Bloodworms, Brine Shrimp and Glassworms all can help him grow his fins back quicker. :)