Preventing Digestion Issues In Bettas

The main causes of digestive troubles are dried 'live' food and bloodworms.

Food such as dried worms etc are reputed to absorb water in the fish's stomach and cause problems and the solution is to presoak them to fully rehydrate them before feeding.
Bloodworms (live or frozen) have large hard heads and too many at one meal can cause a blockage.

However, you haven't named either of these, just standard pellets. You could try soaking them before feeding - and don't feed nearly as much as the packs say. Bettas are well known for begging for food and they will eat till they almost burst if we let them. Since fish can go for days without food it is also a good idea to not feed him anything one or two days a week.

You could try adding frozen food to his diet. Look at daphnia, brine shrimp or mysis. Just one as a pack of cubes will last for ever unless you have other fish as well. You would need to shave a sliver off a cube with a sharp knife for each feeding then return the rest of the cube to the freezer. Feed once or twice a week instead of pellets.