I don't know about sea salt but I don't think you should use it.
There's something called Aquarium salt which you can use as a treatment, it's what I use for Fin rot. If he's not in a 1 gallon tank or close to it, I would get a hospital tank to make this easier on you and him.
Use 3 teaspoons of dissolved salt per gallon. So for a 1 gallon, just 3 teaspoons :) makes it a lot easier. Make sure you condition his water and you'll have to slowly acclimate him to the new water since he won't be used to the salt. An hour or so should be good. Every day you'll have to do a 100% water change and add in the new salt so that the dirty water doesn't effect his fins more.
You can safely use the salt up to 14 days, I don't do it over 10 but it probably won't take that long, maybe up to 6 days. Once you no longer see black or grey at his fin edges you can acclimate him back to his regular tank. Keep up with the water changes weekly, even twice a week or more if it's a small tank. And then you can use StressCoat to heal up his fins. The salt won't heal his fins (I thought that in the first place lol) it will just take care of the rot. So once the rot is gone he'll need help healing up his fins a little more.
New fin growth is white or clear in color and then will go back to it's original color. But the StressCoat helps heal him because it has aloe in it, it is also a conditioner so you won't need to be using the regular one you use. You can use double the dose for StressCoat as well (It should have directions on the back for healing fins)
Hope that helps a bit and may your little guy have a swift healing!