Is your tank heated?
At work we had a betta who had some severe issues with swim bladder and nothing seemed to make him better. He was floating at the top for at least a week or so, but always showed a fighting spirit.
So I convinced my manager to let me take him to my friend's place, where they always have their apartment heaters cranked. They cleaned his bowl regularly and kept him nice and warm. In a couple days he was showing major improvement.
Here's a before and after picture to show you :)
Don't give up hope on him! Try to feed him a bit less, or split up his feeding through out the day. Some bettas are sensitive to water changes, so he may just need a bit of extra help when you do water changes. Keep trying different things and don't give up hope!!