They don't poop immediately after being fed a pea. It takes a little bit of time. It's not a laxative as we know them (a drug that acts almost immediately). It's fiber, and has to work it's way through the system a bit. Give it a little time.
Catfish pellets are entirely wrong for Bettas. They are carnivores, and need meaty foods, or at least pellets designed for them, so that they can have enough protein to survive. Please get him some of those, or you'll just have recurring problems, and end up with a sick or dead Betta.