He is three in November and is potty trained by day but wears a nappy at night as he is not always dry throughout. However he will only poo in his nappy. If I try to get him to sit on the potty or ...

as urine combined with the bacteria in poo can make skin sore and lead to diaper rash. Expect to do it before or after every feed (but only when necessary at night), plus when they've pooped.