you still dont want to give them frozen bloodworms as an everyday thing, it's like feeding us sugar all the time. they should be used as a treat. if he wont eat flakes, try pellets (as it has been said).
i don't agree on "forcing" the fish to eat by starving him until he gives in. i forget who used this example on here, but i really liked it. they said "would you try to feed a baby something they don't like and just starve it until it finally ate it or would you try different foods to see which ones (that were good for it) it would eat."