Awesome. That fish store guy sounds alright, to me.
I'm glad the salt/heat is working, it's much less stressful for your fish. But if he starts moving his gills rapidly, might want to throw an airstone into the tank. Salt reduces oxygen, though it can help the fish breathe, too, if the parasites are in his gills. More air = better for fish, though.
It's essential to do 100% water changes per day and really rinse the tank if you can. Barebottom QT tank is much easier to rinse in hot, then cold water before refilling - flushes out all the cysts and swimming parasites that might otherwise hide in substrate.. if you haven't got a smaller hospital tank, just vacuum the substrate as thoroughly as you possibly can during each water change. Ich is great at coming back, especially if you cut treatment short when all the spots are gone - remember, those white cysts are just one part of its life cycle, and one missed cyst can hatch into thousands of teeny parasites.. hello, re-infection.
The goal is to speed the life cycle up and flush the parasites out before they can latch on again.
Some Stress Guard might be good to add, too, just a tiny bit, to help ward off infection (don't use it if you shift to medication though) and get his slime coat healthy again.