No advice here, since I will absolutely not date anyone who allows a partner to veto another, but just wanted to wish you luck.
Oh, and maybe I do have some advice... be yourself. Don't try to impress - if you're nervous, it's okay to say so. And let them do most of the talking. This is a good opportunity for you to observe how they relate and interact with each other, and hopefully you will gain some insight into how he handles his relationships with women, especially one whom he is letting call the shots for a relationship she isn't in.