S.O.
I would say that he is allowed to get up once after he's gone to bed, that if he says he has to go, then he better go. After that, he has to hold it, etc. He's just testing you now. You're doing right by saying no to him. Once he realizes that it won't work and he actually has to go to bed, then it won't be such an issue later. If he gets up to go, he will have to. But, right now he is just playing a new game and you will have to stop it. My kids do this too when being punished - all of a sudden they have to go. Ha.