What's the temperature of the tank and is the filter maybe too strong (and pushing him around)? Try doing a water change, too, and see if it helps.
A lot of new bettas just don't eat at first, though, so don't worry yet. It sounds like he had been really stressed out.
You could try crushing up the Hikari pellets into smaller pieces, some bettas prefer tiny food. If you get desperate, try live foods (bloodworms, blackworms, earthworms, brine shrimp, fruit flies, etc.).
Edit: You also may want to consider getting a larger aquarium. A 1.5 gallon is pretty tiny and your water parameters won't be too stable.
I don't know what the water temp is since I don't have a thermometer, but it feels normal to my hand. I did a water change earlier, no difference... Do you know where I can find those live foods?