what size tank is he in now? ..
i would suggest slowly increasing the amount of water he's in .. because .. some betta's are bread in 20g tanks .. and cupped immediately when they get color and growth .. so their fin's haven't developed a lot of strength to be swimming in 10g immediately out of the cup and end up biting their tails so they can swim faster ..
so if he's in a 1g .. try to only fill the 10g half way . so he gets used to the extra space .. and grows stronger flippers .. then fill more as he's getting stronger to patrol his space