Sounds like fin rot, as you said. Are you treating for fin rot? If it's not too late and he responds to the treatment, the fins will most likely grow back after he recovers. If the fin rot progresses past the base of the fins and he somehow recovers (not likely if it gets that bad) the fins may not grow back, or may grow only partially.
If you're not treating him already, I'd go on a site like fishyfarmacy.com and check to see if it is fin rot, then go get the appropriate medication, do a big (50% or more) water change, get the meds into the tank, and keep the tank VERY clean and medicated until he recovers.
Good luck.