Okay, so if you're going to use epsom salt for a few days you can pre mix 1tsp per gallon
(16 US cups). Stir it up really well [sometimes it can take a few minutes to dissolve] and use that water for his water change in his regular tank.
You should do this until he begins to swim better.- removing 1/3 of the water and replacing it with more epsom salt water daily.
If he looks a bit bloated he could either be bloated from eating, in which case fast him for 2 days. If you notice his belly gone down, feed him after two days. If you have not fed him enough to bloat him check him for pineconing. Look down from above at him, see if his scales are beginning to lift off (for reference, google image search for "pineconing betta").