Im sorry but my guess is he is just old.
Most of the nice big well developed males you get in
pet stores are already 2-3 years old, they are usually retired breeders.
The only way to make sure you get a young betta is to go through
actual betta breeders.
"Only a fish"- well fish are very complex creatures actually.
They are highly intelligent, have very long memories,
and can feel pain. Flushing a live fish down the toilet is
cruel, as once the fish hits the septic field or sewer, they die
an extremely painful slow death in the toxic waste.
You should always return fish to pet stores if you dont want them anymore.
For sick fish you can properly and painless euthanize them the same as a dog or cat- but with clove oil 1 part to 5 parts water well mixed, fish goes to sleep and dies painlessly. - this is for only very ill or wounded fish you cannot save by medical treatments.
Some fish have been proven to be able to remember a school mate or sibling 10+ years after being seperated from them!
Having kept a lot of fish now, and some very intelligent ones such as
angelfish, loaches, and gouramis, I would say fish are much more intelligent than we give them credit for. Watch them often enough and you can even see them problem solving!
I love all my fish, I cry each and every time one dies. It is no different than losing any other pet for me.