If you leave the child on the potty will he get so groggy he forgets and loses control and just does it. Have actually done that with a child that was to wear underwear at naptime but I had a no pee wear a diaper rule for her for the same reason. Sometimes it mattered how soon after lunch she was going down so maybe he needs to be up an extra 15 minutes before being put on the potty to sit till it is done.

When an older child has an "accident" that I feel is because they just couldn't be bothered they are sent to stand by the bathroom door till I get there - basically becomes the same as time out as they are told they can't play with a peed/poopy bum and to go stand by the door till I can come and change them and then of course I have to make things ok in the playroom before I can leave it so I make sure child has to stand for a couple of minutes - just enough to get my point across.

If they are in underwear they are also expected to be independent in dressing so they must remove the wet stuff and I wash them down, put on the dry pants since they will be harder to get up then leave them with a pile of clothes to finish. While I go do something fun with the others or get out a game I know they like. The idea is to make their life miserable when they have accidents as an incentive to not do it. There are ways to reward without giving a reward. The reward is for staying dry not going.