You can but if he's still got rot, don't use salt again. You should try something like KanaPlex which is Kanamycin and has been used for Fin Rot.
Now are you sure it's fin rot and not just tail biting? Sometimes one can lead to the other but if he's just biting and there's no rot, then using salts/meds will have no effect and probably just make him worse.
At the moment, the best thing you can do for him is to do 100% water changes DAILY until this all clears up which can be a month or so. But don't use salt again, it's much more harsh than the new medicines we have now a day's and can lead to Dropsy in fish.
Can I get a picture to see him, or 5? lol