you can remed after waterchanges, just keep an eye on him while using bettafix/melafix.....
Once they've had fin damage or finrot the new growth is very delicate and is easily damaged or reinfected. I know as my little guy has had it 3 times now, first time was bad and I used medication, the second time he blew all the new growth in one hour, all gone that time I just added salt to his water and did a 50% water change each day, in a week he had new growth......only this afternoon I find him with a hole in his tail, gutted to say the least, so added salt and will see how quick it moves, if it goes fast the will add proper meds..........mine had finrot when I got him but I am also cycling my tank and it does takes its toll on them, Ive just gotten over a massive nitrite spike and I think that is what has caused this to start again.
Keep on top of it and he will be fine, its really upsetting to see them like it though, so I do understand how you feel. In yoru good care he should make a speedy recovery.