You can get epsom salt at any drugstore or Wal-Mart. It's used as a laxative in humans, also as a muscle soak. You probably could feed your guy maybe 4 pellets a day and he'd be fine. Many small meals is best. You didn't drastically overfeed, though, so don't worry too much.
It sounds like he's getting dropsy so I'd suggest starting him on the epsom salt as soon as possible. At this point you may want to put him into a small QT tank, like the petstore cup he came in or a small bowl. Mix one gallon of dechlorinated water with two tsps of epsom salt. When the salt has dissolved, pour that into his QT container. Change the water in his container every day, redoing the salt treatment each time. Don't worry if he's not eating, he probably doesn't have much of an appetite but hopefully it will come back when he shrinks down to normal size.
Hope this helps and I hope he gets better quickly.