Check that your water quality is optimal. What are your ammonia, nitrite, nitrate levels? You should make sure that these are good, and follow treatment on what you're using. If it doesn't work within the time specified, run fresh carbon to remove it, then go with a real antifungal. There's nothing wrong with Pimafix/Melafix, necessarily, but if they don't work, there's no reason not to move up to a different, more potent medication.
Also, is there any redness or swelling around the areas on his side or stomach? You want to make sure this isn't infected.