I don't know too much by way of best pellet brands. I typically feed Freedy a mix of the frog pellets and some free fish pellets I got at a nice fish store. He likes them both. :)
Some bettas are just picky. I've heard of bettas that will only eat one kind of food before... hopefully this will cure him of that. A little hunger goes a long way. ;D
You could try stopping the flakes and giving him pellets only for a while, as long as you're sure he's healthy and can take it. They're stubborn, so it might be a few days. ;D