I looked up Tetra betta mini pellets, and found this listed as the first ingredients: Wheat flour, fish meal, wheat gluten, potato protein, corn starch, soybean oil, corn gluten, shrimp meal.....
A lot of bettas are sensitive to grains (wheat, corn, soybean, corn, etc.). It can cause constipation and bloating.
I recommend switching his food to one that has quality proteins as the first few ingredients. On this forum, the two most often recommended foods are:
--- New Life Spectrum (NLS) betta pellets
--- Omega One Betta Buffet pellets
Petco carries both brands. Petsmart carries Omega One.
Speaking from experience, I have one betta that's really sensitive to grains. He gets bloated and constipated, and ends up doing barrel rolls at the surface. (This freaked us both out!) I switched him to Omega One, and he's been fine ever since.
In the meantime, if you have Epsom salt, you can add a small amount to his tank. Epsom salt has a laxative effect (in both people and fish!), and is safe to use in low dosages. Look for 100% magnesium sulfate. (No dyes, additives or perfumes.)
Dissolve 3 teaspoons of Epsom salt in about a cup of conditioned water. Slowly add it to his tank over the course of an hour. This will give you a total dosage of 1 teaspoon/gal, which should be enough to help "move things along."
I've used this dosage of Epsom salt successfully on my constipation-prone guy. It may take a few days to see the effects.