your betta is most likely still adjusting to his new home if you've brought him home within the past week. give him another day or two and he should settle in.
also, find out what the breeder you got the betta from was feeding, since bettas like to be picky about their food. if he won't eat any new kinds of food, don't feed him for a day or two and try the new food again. fasting a fish for a couple days won't hurt them any. bloodworms are especially liked by bettas, but not to be fed as an everyday food, since they have a lot of protein and too much protein is not good for any fish.
none of my bettas ever cared for brine shrimp or flakes. i would stock your betta pantry with frozen bloodworms, frozen mysis, betta pellets (any brand is good), or the smallest NLS pellets that are sold for small cichlids. you could also keep dehydrated bloodworms on hand, but i don't know if the nutritional content is any good after the freeze drying process. i fed the dehydrated bloodworms for a long time and my fish were healthy, but i also fed pellets, so who knows.
i would feed some sort of pellet as a staple food and give a few defrosted bloodworms or mysis at the evening feed, about 4 times a week. be sure to take out any uneaten food, especially if your betta is in an unfiltered bowl, because it will foul the water if left in. another option is to only feed once a day, and this is good to do if your fish are not overly active or being prepared for breeding (conditioning).
as for the amount of food he's eating these days, 4 or so pellets total on two feedings a day is probably enough for a betta that is in a small tank or bowl and is not going to be used for breeding. the pellets actually swell a little bit in the fish's stomach, so two pellets once in the tummy will take up more room than two dry pellets.
