Mine like both the Hikari bio-gold and the New Life Spectrum betta formula. NLS is a better quality but sinks often so if I don't want to basically hand feed, I put in some Hikari. They will even nibble at the pellet I drop for the shrimp a couple times a week too. Mine get freeze dried bloodworms every few days, I haven't had any picky eaters.
What else have you tried? The bloodworms will give him at least some nutrition for now, but it'd be like us eating nothing but fried chicken. Sure you can survive for a while (and oh man would that be tasty!) but eventually you'd get sick from malnutrition and lack of variety. Granted, humans are omnivores so we have more options, but a carnivore, in my experience, is a bit less forgiving of such situations.