Bettas sometimes do this. if the water is good, it's heated and steady of temperature, and there are no outward signs of illness, then he is probably okay. Be sure do not overfeed..do you fast one day a week? One thing might be he is constipated a bit. This happens when overfed or fed too much high protein like bloodworms as a diet instead of a treat. For constipation, feeding a cooked, shelled pea can work, but my bettas turn their noses up at veggies so i use daphnia, which is insect based. Be sue and do your partial water changes weekly and keep an eye on him. if anything else develops that seems odd, you can e me. Or ask again on here.