Okay, so on September 4th afternoon I got my new fish, Angry Russell the Misunderstood Betta, Angry Russell or just Russell for short. I want to help him, I don't want to be a bad mommy

Also, I don't know if it matters but I got him at Petco and I don't know what they feed them there. He had a lot of stuff on the bottom of his container though but I'm not sure if it was uneaten food or just poop (I think it was poop)
The day I got him he not even acknowledge his food. Today he took a bite of one pellet and spit it out several times until getting swimming away. It seems he got a LITTLE food off the pellet but not enough to keep him sustained. I have been switching between Omega One Betta Pellets and TetraBetta Floating Mini Pellets and he doesn't seem to have shown a preference. He has a lot of energy and I know they can survive around 2 weeks without food.
If he does not eat his pellets, should I leave them floating on the top of the tank so he knows they are there and is comfortable with them as long as I keep the tank clean? Should I try, say, feeding him something like bloodworms or something else that you suggest after another day or two? Any other ideas?
I posted a picture of him and of his tank. It seems to have enough to stimulate him but he keeps just swimming back and fourth the back two walls, I think he's stressed out. He's also spitting out a lot of bubbles. Any suggestions on how to keep him happy or should I just give him more time to adjust?
Thank you for any help
P.s. Sorry the photos of him are blurry. It's hard to get a good picture of a moving betta on a phone camera
His tank setup: