You can try giving the fish an epsom salt treatment. Epsom salt is magnesium sulfate, a gentle diuretic that should help decrease the bloating and get his digestive tract moving again.
In a separate container, prepare water of the same temperature, and mix in one tablespoon per five gallons of pre-dissolved Epsom salt. Acclimate the fish to the salted water for at least a half hour. Alternatively, if you used water from the betta’s tank, you can transfer the betta into the water directly and dissolve the salt in another container--slowly adding the dissolved salts to the water over the course of at least a half hour. The duration of this treatment is three to five days--during this time, do water changes as necessary, replacing the dissolved salts as necessary.
Hopefully this will inspire him to poop.