Anyone have suggestions to transition a 4-year old boy from night time pullups to underwear for bed?

Once we got him potty trained we put him in pull ups for bed time. We tell him if he keeps his pull ups dry three nights in a row he can wear his big boy underwear to bed. But he ends up peeing in them during the night or in morning instead of going to the potty. Anyone have any suggestions or is this something that naturally just takes care of itself? Thanks!