I have the same problem with my guy Ghost. He's a lovely cellophane who hasn't eaten for almost 3 weeks now!
He did, on two occasions, eat some frozen bloodworms (properly defrosted, of course) and spit them out soon afterward. I have no idea how long he can survive this way!
I bought 3 kinds of pellets, 2 kinds of flakes, frozen bloodworms, frozen brine shrimp, and now I'm hatching some brine shrimp eggs so maybe he'll eat some live food. I know baby bettas eat live brine shrimp, so maybe it'll remind him of simpler times, back when he was just a little fry, and he'll start eating.
If I come across something that works, I'll let you know. If you come across something that works, feel free to post it or message me! We need to stick together with out finicky little friends!!