Do you have a picture?
I've had picky
bettas in the past. I never found a good remedy for it, but it's good that you're trying various foods.
However, stress can cause strange behaviors in bettas. You said that he can see his reflection in the back of the tank? I would try to block the back wall as much as you can with decor. My current betta overflared his fins many times because of this same issue. His fins ripped because of it, and he got fin rot because of the stress of constantly seeing his reflection. Finally, he developed tail biting (may be related, may not be). He is now in a low light, blackwater tank with heavy decor (live and silk). It took a lot to destress him, but he's very happy now.
You may just have a high strung betta and need to get to the root of his stress. What size tank is he in?