Hmm I had this issue with my two females when I first got them. They wouldn't eat for a few weeks. I switched to Hikari Betta BioGold, and they really seem to like those. Now they can't wait to eat! They hated the other food I got for them initially. I've tried feeding them the old food on occasion and they still refused them.. so maybe try buying a few foods and seeing if any of them seem to please your little fish more. :)
My new Betta doesn't want to eat either. A lot of things I've read say that they need time to adjust from the transition of store to tank. Hang in there <3 I hope he starts eating soon. He really is very pretty.