Hope this doesn't sound too harsh, but get over yourself. You didn't want to give him enough to be his one and only, he found someone who wanted to be his primary, and who, frankly, he wants as his.
Yeah, he probably will grow to love her more intensely than you. Yes, he probably enjoys her company more. But, then again, you have others you prefer spending time with, as well.
But that doesn't alter your and his relationship. Sure, you might want to clarify your position for the sake of all involved. But your and his relationship should be strong independently
Take it for what it is, for what you've created, and be happy. For you, for him, for her. Eventually, she'll want to meet you, don't push it, let it happen naturally...if she doesn't, that will likely be a problem for him and her to work out (if she can't stand him having a relationship with another woman, poly isn't the lifestyle for her, anyway).