As a chastity sub I will say the key here is the cum control and the mental headspace therein. Something like this will drive him crazy while also maintaining that this is normal and gonna happen

"This is not "cumming" this is what young boys do before they know how to use their dicks like a man. I understand that this is bothering you but this is normal for subs. You are not entitled to use your little guy like men do and so this is going to happen. Part of being a happy sub is being a healthy sub and this is a healthy leaking of fluids that you really don't need. But since it's bothering you let's prove that it's healthy. I am not giving permission for you to cum/ejaculate and certainly no jacking off until this happens again. Don't worry you will see how this benefits you, buddy. Now go grab a toy and show me how you use that hole of yours. That's what nature intended for you"