my baby is 3yrs 4 months old.. he poops only in diaper n that too in a particular room.. apart from pooping he doesn't wear diaper at all.. not even at night... how can i change this habit of pooping in diaper.. n train him to poop on a potty chair.. he is very phobic to sit on the potty chair n avoids n very resistant to sit on it.. n even while trying he is unable to put pressure on the chair to poop.. kindly guide..