Without his side and hearing only what you have to say, I'd feel as if he were in it for the sex as well. Hanging out, whether as couples or in a triad, should be about more than just having an available space to have sex. Right now, it seems as though that's what he's most focused on.
If the three of you have already made plans to do the drive in and sleepover at your house, then why not do it-with or without him. I'm not trying to say cause drama or not include him, but why should he have the ultimate veto on a group decision? If she still wants to do this, then do it. And let him know he is not only welcome, but you'd both love for him to come. If he doesn't, he excludes himself and that's his own decision. Don't let the relationship start off with one person calling all the shots and one person sacrificing more than others. If it bothers you now, think how much harder it will get as time goes on if this pattern continues.
I'd rather be hated for who I am than loved for who I am not.