this might be a stupid question, but how long have you had your betta? he may be suffering from old age?
I had tried some peas previously, but he wouldn't take them. I just tried again, and he did eat some this time. We'll see what happens!I would feed him a pea if I were you like someone suggested earlier, it may help. You can buy canned peas, just make sure they're plain and unsalted. Just take a pea, remove the shell and squish a little bit of it to put in the water for him. Bettas can get easily constipated, and although his problem may not be constipation, it may help for him to have a little bit of veggie.