Clean warm water is best medicine for healthy bettas.
Is his tank filtered and heated?
How often and how much water do you change?
They can rip fins on sharp things in tank(fake plants,rocks,wood).
Some use a panty hose test; with panty hose on your hand rub things in his tank.
If they catch the panty hose they can cut his fins.
Your pics look like nice live plants?
I see the heater also now(78-82F) is comfy!