Hi lex, I'm glad he's trying to swim around more. It sounds like he still has some swim bladder problems. Is he bloated?
I would get the Maracyn-II to have on hand, it is invaluable for sick bettas. For the moment, it sounds like clean water has helped him a lot so I actually want to try an epsom salt treatment first. The dosage is 1 tsp per gallon to start with, pre-dissolve it in tank water before pouring it in. Continue with the water changes and redose the salt as necessary. If he hasn't shown any improvement after 5 days but he isn't getting worse, up the dosage to 2 tsps and continue for another 5 days. If at the end of this time, he is still struggling with "floaty butt" then we'll try the Maracyn-II.
Melafix contains an ingredient called tea tree oil, which is beneficial for humans and most other fish. But bettas have a special breathing organ called the labyrinth organ that allows them to breathe air from the surface and tea tree oil has been known to damage this organ. All fish with labyrinth organs, including gouramis, are at risk from tea tree oil.