Hmm yeah, Tetra's not know for good quality. However if that's what you got, then that's fine for now
Some Betta's are just extremely picky eaters, as I pointed out, all of mine are now haha.
I don't know if you were aware or not (sorry if you are!) that freeze-dried anything is bad for you boy. Yes, it's high in protein but it promotes constipation in your boy which might be part of why he's not eating. Same goes for the flakes. However you can feed the Freeze-Dried BW's as a treat each week, maybe 2-3 worms. but no more than that unless you risk constipation and bloating.
I know it might not be feasible right now but I really do suggest you get New Life Spectrum or Omega One for him. The other brands of food, I can almost guarantee all of them have some sort of filler in the first three ingredients which is what you mainly look at for Betta food.
Fillers are anything like wheat's, grains and other corns. Those aren't good for Betta's, so the things we look for are Whole Salmon, Krill, Shrimp, Fish, all that stuff is good. And in the first three ingredients as well.
So just something to think about