Thank you sooo much OldFishLady =D To add on to what she said, tannins are the extracts (probably not the right word, can't think of the right one right now lol) of either Indian almond leaves or dried, crushed oak leaves. You can also use decaf green tea (just green tea, no added flavors), but it's not quite as strong as the IAL or oak leaves. Like OFL said, add the leaves (if you can get them) and the 3 tsps of salt into a gallon jug of dechlorinated water (use the stresscoat as the dechlorinator). The water will get nice and dark from the tannins, which is normal and is actually a big part of what helps the betta. Milk jugs work great, if you've got one!! Try to find the tannins if you can, as the salt is irritating and the tannins will really help soothe him and keep him calm. But if you can't find any, the stresscoat will still be of some help and just keep is tank nice and dark and quiet.
Edit: nevermind, i was writing while you posted!!! I'm sooooo glad he's feeling better, and it looks like the fuzz is already gone!!! Just continue with the salt and stresscoat for a few more days and see how he does, it looks like he's going to make a full recovery =D