Epsom salt for buoyancy problem, sbd, it also helps to purge parasites out, clean digestive system. But if he don't have problem to poop and if he is not bloated you don't need to use right now.If he will become bloated it one of the symptoms for internal parasites. Epsom salt will not hurt the betta and can be used for really long time, and some times even indefinitely with fish that has chronic sbd.
So see his next poo . Bettas poo will be the color the food you feed them, but the food was not white. I think he might have internal parasites so just watch next poo and if it white, clear,stringy,wormy shape i would treat him. But epsom salt along will help but will not treat internal parasites. So if you can just have medications in case you need it. And i do not recommend to buy TetraTetra Parasite Guard . It has a few ingredients that are not fish friendly. I think someone once said diflubenzeron was even used in lawn fertilizer. If the dose was just a little off, it's possible the medications burned him.
Epsom salt you can use 1tsp/gall up to 3tsp/gall with daily 100% water changes. You need to pre mix it and make sure it dissolved first. And every time that you change him introduce him slowly to the new water, by adding a little bit in his changing cup (2-3 times) and let him acclimate for about 4-5 min in between.