Hi Wany and welcome to the forum.
Don't worry about the pea. Some people are adamantly against it, some say it's okay. I don't think it does any harm as long as it is prepared right and fed only once in a very small amount (no larger than a pellet of food). If you choose to use the pea method again, be sure to use NO SALT ADDED peas. However, a much better alternative is frozen daphnia or frozen brine shrimp.
The reason you want to use a pea or epsom salt treatment is not to treat swim bladder disease/disorder but to treat constipation. When a betta gets constipation the, er, matter builds up and pushes against his swimbladder so he can't control it. The fiber in peas acts as a laxative and epsom salt is also a laxative solution (even humans can use it as such, but it's disgusting to drink).
Can you describe the color of his poo? Is it clear or whitish? It's not uncommon for poos to hang there for a long time. Disgusting but common.
How big is his tank? If it is over 1 gallon, you may want to treat him in a small QT container. I will go on the assumption that you are moving him into a separate QT container but the dosage remains the same if he stays in his tank, 1 tsp per gallon. You are going to want to do an epsom salt treatment. In a separate container, mix up 1 gallon of dechlorinated water and 1 tsp of epsom salt. Let the salt dissolve, then fill his QT container with that. Save the rest of the epsom salt water. For the next five days, do daily water changes and redo the salt. If he doesn't appear to be getting better, up the dosage to 2 tsp per gallon and continue for another 5 days. If he has not improved by then, post back and we'll see if he needs medical treatment.
Your tank is fine without a filter or aeration, but it does need a heater. Bettas need warm water to stay at their healthiest, about 78-80 degrees. You're also doing a great water change schedule as long as the tank isn't under 1 gallon, please keep it up.
In the future, don't listen to anything your pet store tells you about bettas. Chances are it isn't true. While bettas are really quite easy care, they do require more care than they tell you at the pet store. The pet stores would have you believe you can toss a betta into a tiny vase and it will live off the plant roots in there. NOT TRUE. If kept in clean, heated water, bettas are very easy care and also wonderful, personable pets.