ok, I am not quite sure what he has, but you might want to follow these steps for a few weeks:
~ Clean his bowl 50% in between changes.
~ put a pinch of aquarium salt in his bowl every time you change the water
~Make sure to keep his water Very clean
~ You may want to try giving him some frosen bloodworms occasionally. (You don't have to) My betta always seems more refresshed after one!
~Hope for the best! Tell me if you see any other sighns of sickness such as greay patches, flacking of scales, rubbing against rocks, etc. I think that Your betta might have swimbladder disease, but I am not positive, it might be something else. It might be early signs of Finrot which is very easily treated if caught early enough. Also, If his immune system is low, it is a good idea to remoove sharp objects from his bowl because if his fins are ripped he is more suseptible to infection, or parasites. I hope that I helped. Personal message me if you have any urgent questions, if not just write them in this thread! I hope your betta gets better!