What a handsome boy with gorgeous color!
That looks like fin melt to me which like most problems is due to poor water quality.
Here is what I would do:
1) pre mix 1 tsp of Aquarium Salt in 1 gal of conditioned water in a clean jug
2) add Indian Almond Leaf (IAL) if you have, few crushed leaves to be steeped 30-60 minutes to release the tannins
3) add API Stress Coat or Seachem Stress Guard if available
4) then pour into his current container and add him
5) repeat the above steps daily and observe for 5 days for changes
6) fast for couple of days (for the floaty butt), he looks healthy otherwise so no need to worry about skipping couple days of meals to see if this goes away
7) varied diet including high protein like frozen bloodworm, brine shrimp etc will help strengthen and growth of the fins
HTH! Basically super clean water is what he needs now. Good luck with your betta and keep us posted.