i would say probably use raw chicken, but give it a wash first to get any blood off(unless its very, very fresh) as the blood tends to go stale pretty quickly
Lol... not all bettas are going to love Hikari betta pellets, just like not all humans are gonna love pizza.... even though they're insane! Mmmmm... pizza.....
Anywho... maybe he wasn't fed pellets when he was at the petstore? and is more used to the flakes.. *shrugs*