Congrats on your (daughter's) betta, jsoong!
New bettas can be very picky, if you are feeding them a new food than what they are accustomed to. If he is only eating one pellet a day, then he's still being finicky. It took one of my new bettas a whole week to eat anything!
I would continue to offer the pellets. He will become "used" to them. I give my bettas 4 pellets a day in the morning. In the evening, I give them 3-4 freeze-dried bloodworms - just a little bit, and if I miss a night it doesn't matter. Sometimes I alternated the bloodworms with a little bit of freeze-dried daphnia at night. Live brine shrimp can be a special treat. It shouldn't be a staple food because there is no nutrition in it - it's like us eating potato chips all the time. It is fun to see how the live brine shrimp get the betta's attention, though!
Tetra makes a betta flake food. If your betta will eat them, you can alternate the flakes with the pellets also. Some of my bettas ate the flakes and some didn't.
Be sure to feed a pea once a week. I boil frozen peas in a glass measuring cup in the microwave (frozen peas are less messy than canned peas). Let cool, peel the skin off and throw it away, and chop the "meat" of the pea into small pieces for the betta. If the pea pieces are about the same size as his pellets, he should accept them. I use a little paring knife to cut the pea (yes, it's tedious - what we do for our fish!!) and feed about 4-5 little pieces.
I feed the peas on Saturday and on Sunday, the fish are fasted. On Monday I resume their normal diet. Be careful not to overfeed - don't let the betta's little stomach get too bulgy! Some bettas are prone to bloat.