A.S.
Would some counseling help him? Perhaps he still has some unresolved issues from his divorce.
He's right - you don't understand. But that's a double-edged sword. Because you are not in his shoes you are probably able to be a bit more objective sometimes. If she seems excited and happy to go then he needs to accept that he has to let her grow up a bit and have her own life.
I've gone through this with my husband (I am the bio parent and my husband is the step-dad). He has given me good advice at times, primarily because I am so close to my son, and so worried about him, that I can't always see the forest for the trees. My husband doesn't totally *get* it, but over the years I've learned that he's not always wrong. And it helps to have a guy's perspective when I'm dealing with my own "guy" growing up (teen son).
Good luck and hope you guys can get to a good place.