Is his tank cycled, filtered and how long has he been since a water change? I would do a major water change with conditioned water...with the water conditioner in it that removes the chlorine or chloramine (at least half of his water) and make sure that his water is staying at around 78 to 80 degrees Fahrenheit or 26 degrees Celsius. Also if you have been feeding him pellets please do not give him any food at all for at least a whole day and see if he is constipated. If he has been getting bloodworms then he needs to have a fast also but can have peas if he will eat them.
It really sounds like he is starting to get swim bladder problems from either cool water or overfeeding. So cut his food out for a couple days and do the water changes and keep him warm at least within the limits listed above. He is going to need super clean water to recover and that may mean daily water changes if his tank is not totally cycled. Bettas do need to be in cycled tanks and if his filter has not finished with the cycle he could be suffering from ammonia toxicity.
YOU saved your little friend. You just needed some advice, but you were the one who did the work to help him. Don't forget that. He will know that you gave him the help and he will appreciate your help.
A forum community dedicated to Aquarium owners and enthusiasts. Come join the discussion about collections, displays, product reviews, accessories, classifieds, breeding, health, behavior, housing, care, and more!