Aquarium salt is not suggested in cases of popeye because of the fluid build up behind the eye is actually causing it to pop eye. Epsom salt will draw the fluids off, while aquarium salt might make it worse.
Epsom salt (like aquarium salt) should only be dosed once. It doesn't break down or even evaporate. You only need to add more along with a water change. The Maracyns on the other hand need you to keep up on them and not skip treatments. I suggest then that you do not do antibiotics until you can be home and be fully committed, but start epsom salts and increased water changes, remove filter etc. Right before you leave do a 100% change and do another when you get back.
There's a good chance he won't need any more than epsoms and clean water anyway.