I'm not a betta expert, so I'm not SURE about this, but it sounds to me like:
a) He isn't really hungry
b) He doesn't like the food you are feeding him
c) A combination of both
I've seen a lot of fish do this mouthing/spitting behaviour over the years (mostly goldfish). With goldfish they do it a lot with poop. It's like a 'hey, this looks like food-- ewwwww' reaction. This is what leads me to think that he's not a fan of the food. You could try switching to another kind and see if the behaviour stops-- there are a ton of options out there. Alternately, try feeding him a little less than normal. It's a frequent problem that people overfeed their fish. They have tiny little tummies, so they don't really need as much as a lot of people think. Also, I know a beta breeder who says it's not a bad thing to NOT feed a beta for a day. This helps clean out their system, raise their activity level, and increase their appetite for the rest of the week.
Just some thoughts.