Get the liquid test kit as it will give more reliable results. You need to test for ammonia as well.
You have two options. Return him to the tank and cycle the tank with him in it. However if you choose this method you would need to check parameters and do water changes frequently to keep ammonia and nitrite below 0.25ppm whilst it is cycling. Also, for salt baths you will need to transfer him to a separate tub as salt baths should not be done in the main aquarium. The risk of this method is that if not diligent enough you may expose your axolotl to further ammonia and nitrite which will not help his recovery.
The other method (which I recommend) is to keep him isolated in a tub and do 100% water changes daily in the tub until the tank has cycled and he has recovered. This means you can use the fishless method and not risk exposing your axolotl to further ammonia and nitrite and you can monitor his condition more closely.
To do the fishless cycle, you can place the old water from your axolotls tub in the main aquariums each day when you change his water. Food can be used as an ammonia source but this method may foul the water as the food rots. If you do use household ammonia you need to use pure ammonia and not any ammonia that contains other soaps or chemicals. To measure to 4ppm you need to test the water for ammonia so again the first step is getting a liquid test kit that also has a testing bottle for ammonia. A little bit goes a long way so just add a few drops, wait half an hour and test again to see where it is at and add more drops as needed to get it there. But read both articles thoroughly as they contain a lot of good info.
Here is a link on fungus that you may find useful as well
http://www.caudata.org/forum/f46-be...-axolotl-general-discussion/96018-fungus.html