Sounds more like sitting on a potty (get one that is more chair like) should be where time outs occur and then if he pees it goes where it is supposed to. Maybe sitting there in wet underwear for awhile won't feel so nice.

This is one of the reasons I am in no rush to get daycare kids out of pullups. I don't care when they move to underwear but I will not let a kid control our day and that is the power we give them when we take diapers away. It is a power they don't get at my house till they are mature enough to use it properly. A privilege that can be taken away.