Maybe he is tail bitting which can be mistaken for fin rot. What tail type is he? Sometimes the heavier fined ones can become bitters. Some bite because they are stresed or angry. Some because they can or they are bored.
I don't think AQ salt helps regrow fins. I think it helps combat infections from the open wounds. Stress coat is good to use for fin regrowth and is also used for water conditioner.
I suppose it could also be from ammonia burns but my guess would be either bitting or fin rot.
Do the fins look like this? If so then he is a tail bitter.