I would.. start by giving him 1 piece in the morning, 1 in evening.. that way he recovers from the fasting and gets used to his new food gently. After a few days of doing that I would feed him 2-3 pellets of the buffet twice a day, as they are small and shouldn't cause any digestive problems.
I just read the ingredients on the Hikari...I must have gotten the new recipe, it's all wheat germ and dough-y stuff. When he feels better and I stop fasting him, should I switch him all the way over to the Betta Buffet?
Hikari had recently changed their ingredients from what I understand, and actually went down in quality. Unsure if maybe last time you bought it, you got the new recipe which could affect him. I use the Omega Betta Buffet with great success on even my pickiest of eaters.
I use a gallon jug when I change their water, so I can do that easily :]
Should I give him 100% water changes without the salt to keep it really clean? It'd be easy since I lowered the water to only a gallon.