I've read between 1 teaspoon per quart to 1 teaspoon per 2 gallons to 1 tablespoon per gallon; unfortunately there doesn't seem to be any set ratios.
I'd start with 1 tsp/gallon first. Take some water directly from the tank and put it in a clean container. Add the epsom salts in the 1 tsp/gallon ratio (for example, a half tsp if you only have a half gallon in the container). Let the fish swim in there for 15-30 minutes. Keep an eye on em; if they appear stressed take him out after 15 mins. They should poo either during or shortly after. If he doesn't poo, and doesnt appear to be stressed, try a stronger ratio of epsom salts to water tomorrow (maybe a tablespoon per gallon). I'd do this over a course of a few days, as epsom salts not only have a laxative effect, but also reduce accumulation of fluids and if I remember correctly, your guy is somewhat swollen on the one side.