Does he spit and then retake it, or just leave it?
Definately try something else, TetraPro is good but there is alot of specialist food for Betta that i have no clue about.
Another thing to remember with Betta is that they should (IMHO) be given at least one fast day a week and at least one day where they receive pea and no flake. I treat my fish with live foods at least once a week, the rare prawn (careful feeding prawn) and a number of different prepped foods.
Poor nutrition / vitamin deficiency can lead to a number of different illnesses so try and vary his diet as much as possible.
In addition fish can lose their appetite in response to illness (nothing specific, just illness in general) so keep an eye out for any seemingly odd behaviour.
If emaciation does occur then you likely have a problem, but it doesn't sound like you need to worry about this at the moment.....