I understand how painful it must be to be in a loving relationship that is not acknowledge publically. I can hear the hurt in your post.
The dynamics of a sub/dom relationship can be a challenge for someone outside the life to grasp. It's almost like an alternate universe. I'm a sub and my dom and I have profiles on there. I'm collared so I'm exclusive to my dom but that doesn't extend into my vanilla life. Certainly not for him either.
It sounds like you would like him to tell his sub of your existance. I understand the comment about "controlling the flow of information". My dom does that too. It's part of our relationship. I don't in any way think that he is hiding anything. It's just part of the dynamic. Perhaps they are not yet at the point in that relationship where he is ready. How long has she been subbing for him? If he is actively training her then it could harm the development of their relationship. He isn't necessarily lying to her.
Just try to remember that his dom relationship with her is different than what you two have together. My Dom is a completely different person outside of a scene. He thinks differently, he acts differently.
This is absolutely not a reflection on the importance of you in his life. Please remember that.