J.K.
The good thing is that you recognize your feelings and also consider his. You're probably right that he's trying to not hurt you and isn't aware that the Maybe is killing you inside.
I think it is time you just bluntly say to him - "Right now, let's decide if we're ever having a third. There can't be a Maybe. If we're not going to, I need to know for sure so I can start working on getting over it."
If you get your solid No and still feel resentful after a reasonable period of time has passed (you'll go through a grieving process) then you'll need to talk to someone that can help you move on. Remember to keep the focus on what matters most - your 2 existing children. You don't want negative feelings to eat away at your marriage until it falls apart. That isn't what is best for your kids, which I'm sure you already know.