If it is constipation, fast him for one or two days and refrain from feeding dry foods (pellets, freeze dried, flakes, etc) at all. Just feed little bits of pea and frozen BW or BS.

He could also be depressed? Does he get to see Siam much, or do you hold a mirror up to his bowl often? Some Bettas do get depressed without daily sight of other Bettas ~ have you tried that to cheer him up a little?...