Give him a pea. That should help with the constipation.
You can prep it by pulling a frozen pea out of the freezer, putting it in a small microwave-safe bowl in dechlorinated water to just above the pea. Then put it in the microwave. 10-15 seconds should do it, then pull the pea out and stick it in room temperature dechlorinated water. When the pea has reached room temperature, remove the shell by compressing the pea, and feed the innards to the Betta.