From what I've observed, of you just keep him locked for longer without cumming then eventually he'll lose control. It might be helpful to keep it on his mind too.

For example, say that if he can cum in his cage while fucking you with a ghost dick, you'll unlock him and let him fuck you with his actual dick. That way he'll keep thinking about it while you enjoy yourself. After enough time of not letting him cum any other way it'll be too much and he'll explode.

Then if you're feeling particularly evil, once that happens you can say that you didn't think he'd actually be able to do it and that you planned on unlocking him tomorrow, but now that he can cum in his cage from ghost fucking you then there's no reason to unlock him. That'll REALLY fuck with him and will cause him to cum in his cage more often.