Feeding him the pellet is fine. He pooped already, so it's OK to give him something to eat.
I would keep him in the Epsom salt solution for now. It will reduce any fluid retention he still has, and should help with buoyancy issues.
(In answer to your question: It's OK to keep him in a low dosage Epsom salt solution. (I generally give the 10-14 day recommendation, so that people have a known time frame. However, some fish have chronic issues, and need to be kept in a low dose for longer periods of time.)
Also, I don't recall what food you're feeding, but it should be one that is high protein. On this forum, the two most frequently recommended foods are New Life Spectrum Betta pellets and Omega One Betta Buffet pellets. They have fewer grains than some of the other foods, and bettas that are prone to digestive/bladder issues seem to do well with them.