If he still isn't eating in a couple more days I recommend trying the frozen pea method. Take a frozen pea and thaw it out (dropping it in hot water and letting it soak works well). Then remove the shell and cut the pea into small Betta mouth-sized pieces. Drop a piece or two into the water where the Betta can see it. Many Bettas will be enticed into eating a piece of pea or even multiple. This can help his appetite return to normal.
If you add too many pieces of pea -- particularly if he doesn't eat them -- you can cause a spike in nitrate and nitrite levels as the food rots, so do be careful.
Frozen peas are recommended by many as a regular additive to a Betta's diet, too. It can help prevent constipation, which can be a serious concern for Bettas.