Bettafix definitely has mixed reviews. Some say it works wonders, others say its does nothing, and some say it makes things worse. There is an ingredient in Bettafix that is supposedly dangerous for labyrinth breathing fish, but I think it's diluted (0.1% or something) enough that it's mostly safe for bettas. I would use Bettafix as a last resort, if nothing is healing his fins.
Have you tried a little bit of aquarium salt in his tank? This really helps stave off infections as well as promote fin growth a bit. My betta ripped up his fins a bit on a plastic part of a silk plant, so I added a little aquarium salt (1 tablespoon per 10 gallons, some people do 1 tbs per 5 gallons but I was playing it safe). It's been 4 days and I see new growth and there's no infection.
Also, you could try Indian Almond Leaf or Indian Almond Leaf Extract. You can order these online. The leaves have an antibacterial/antifungal property and do wonders for betta fish's immune system. Breeders in Thailand swear by them.
Most people recommend warm and VERY clean water for betta's injured fins with some added aquarium salt. Try this first before you add any medication.