He can sense the urge...he's choosing not to do anything about it. I would say he is holding his bowel movements all day and waiting until at night. Would this seem to be a correct assumption? Is he indeed having bowel movements throughout the day or is he only going at night? If he's holding it during the day, then it would make sense that there is some sort of "fear" on his part to have a bowel movement until it's inevitable, which would explain why he is going at night. I would suggest allowing him to go without underpants, shorts, or pants during the day and encourage him to have a bowel movement during the day. If he "refuses" throughout the day and continues to only have one at night, then at bath time, before he gets a bath, I would put him on the toilet and tell him he has to go poopy before his bath. I would not allow him to go to bed until he has had a bowel movement. He needs to learn not use his underpants at night for any type of potty activity. Usually, when a child is without pants on when the urge comes on, they will most likely need to do it someplace but if pants are on them, they will use those pants be it underpants or shorts/pants to do their activity. Rest assured, he knows he has to go. He is simply refusing to go and instead using his pants instead and to be totally potty trained, he needs to understand that he must not use his underpants to use the bathroom. Keep doing the rewards. It makes a difference and you might want to try something different at this point...a different reward. They sometimes get "bored" with the same old reward. This sounds to be bothersome to you and vamp up the reward with something that will make it more encouraging for him to keep his pants clean even at night.