I am not sure, i read that some bettas prone to have this problem, but if you saying it started right after you switch to Omega then i would just stop feeding him new pellet and go back to tetra betta plus. Frozen daphnia helps with constipation but it also good to feed with any way since it has a lot of protein. I am not familiar with Omega pellets. But you can try New Life Spectrum Betta formula (everyone on the forum recommending it)or AqueonBetta pellets, i also read and tried myself that you can feed with cooked shrimp (size of the pellets), brine shrimp. I feed my bettas with 3 different brand pellets , shrimp, freez dried brine shrimp and they all live long life:)
I think with this problem really better less water movement. So make sure filter is not too strong in his 10 gall.
Just in case if it will happened again, and for long time so you know you can use Epsom salt indefinitely. If you use it on regular basis then you don't need to do daily 100% water changes though.
If you cycle your 10 gall you need to do 50% twice weekly with gravel vacuum and filter swish/rinse a few times a month. If you decide do not have a filter then you need to do 50% weekly with gravel vacuum and 100% every 2 weeks. Well it what i would do ..