I use Epsom salt with all my Bettas with no ill effects and I have over 200 Bettas, 100 are new fry, 32 at age 92 days; and I use 1 tlb epsom salt per 5 gal of water, I have never had any problems with any type of fungus. Epsom salt are also good for constipation and swim bladder problems but at higher doses for shorter times. Changing at least 50% of the water daily will also help, dirty water can be deadly for fish.
Are his fins fuzzy? are you sure it is fungus

how is he eating

how old is he??
Most state that life span on a betta is about 2 years; but I have 2 males and 3 females that are pushing 5 years to date...good luck