An antibiotic bath is basically just having the antibiotic in the water and leaving the fish in the water to allow the antibiotic to be absorbed through its skin as opposed to injection or food administration, so adding the medication to the tank water is in effect giving him an "antibiotic bath". The salt bath is recommended at a dosage of 1 tsp per gallon of water built up gradually to prevent shock. Be sure you are not using table salt as it has additives that are not good for him. You will need Kosher or Aquarium salt. There are antilbiotics that are fairly strong that are recommended for an antibiotic bath for the treatment of finrot, they are Chlortetracycline, Oxytetracycline hydrochloride, and Nifurpirinol. One commercial medication that contains some of the above type medication is Furan-2 put out by Aquarium Pharmaceuticals, Inc. The directions listed in my fish health books say that the antibiotic bath needs to be continuous for 5 days and then may need to be repeated. These are potent medications and used as a next step after the Melafix and Pimafix. If you decide to use this medication it can be used with the salt in the tank as well.
I am so sorry he is having this trouble. Please let me know if there is anything more I can do. Believe me I do know what you are going through. There is no pain like seeing your little friend suffer and not knowing how to fix it. I will be sure to watch for your posts most closely.
Rose