Nitrites look perfect! I would keep checking water changes for any future spikes.
For your betta, the bulge youre talking about is normal on bettas. The "bulge" is their swim bladder organ. Its normal for bettas to have a small bulge on both sides, however it does seem bigger than normal. Doesn't exactly mean he has SBD just yet considering he is able to swim still.
Are you providing him the right nutritious diet bettas need? They need a huge variety of live bugs/larva,
brine shrimp, daphinia,
bloodworms, dried bugs etc. If you solely feed him pellets or flakes, then he will for sure end up constipated, then leading to SBD. Are you also pre soaking his food before feeding? If not, the food will swell up in his stomach and cause constipation. Its also crucial you fast him 2x a week to make sure he fully digest everything and prevents any buildup.
I would fast him for the next 3 days, keep up with small WCs, and add in some concentrated Tannin Tea extract to help with stress, fin rot, and provide
antibacterial properties.