welcome to Fishlore.. Bettas can heal up well from something like that, just as the previous posts suggest, you want to protect him from infection and help his system be strong and regrow the fins.
Here are the products you'd want:
Vitachem (unless you're lucky, you'll have to order this online):
http://www.drsfostersmith.com/Produc...8&N=2004+22763
It will help regrow fins better than anything i've tried.
Prime:
http://www.drsfostersmith.com/Produc...&N=2004+112994
This is a favorite water conditioner that also helps lock up toxins and enhance slime coating.
Fish Protector:
http://www.novalek.com/kordon/fish_protector/index.htm
This is a super product for healing up a fish! It'll probably have to be ordered online too.
Triple Sulfa:
http://www.drsfostersmith.com/Produc...&N=2004+113021
Just in case he shows signs of finrot, you'll want to have a box of this.
Rid-Fungus:
http://www.drsfostersmith.com/Produc...&N=2004+113021
When we treat for finrot, we use this with the triple sulfa. It's another "just in case" item.
Just keep his water around 82 and extra clean with water changes and make sure the current isn't too strong for him. As soon as you can get the vitachem and fish protector, start that for him. Make sure nothing in his tank could snag his fins. We use flat glass marbles instead of gravel. (more cleaning maybe, but smooth on those long fins) and check decorations to see if anywhere inside or out could snag pantyhose (ie: betta fins).
That's about all the basic advice we can think of.. we brought home a betta pretty torn up and now she has the most beautiful fins.. so your guy has got a great chance at total regrowth!
