Practice the silent diaper threat. When he's acting up like a baby, or just when you want to fuck with his mind, simply take out a stack of 2-3 diapers, baby powder, and locking panty covers and set them on the dresser in your bedroom where he can clearly see them. Don't say anything, just let him imagine (and fear) why you have set them out without saying a word. If he asks you, tease him that he knows "exactly" what they are for and that just asking you might double the size of the stack of diapers.
Instead of telling him when you will let him out of diapers, train him to see that whenever there is a diaper out on the dresser in visible site, then that diaper will be used no matter what before he's allowed out.
You can then fuck with his mind by setting out multiple pairs of diapers in a stack to threaten him or reward him by taking a couple off of the stack and putting them away where you keep them.
You can also put a diaper out at the last minute, just before he thinks he's about to be let out and he has his hopes up high.
If we whines, complains, or even asks you about when he will be allowed to stop wearing diapers, simply go to the drawer and automatically pull out three more pairs and add them to the stack. Then look at him and ask him if he’s learning, or if he wants to see what happens if he continues to pester you.
Be strong, but sweet.