G.T.
This is kind of like that post yesterday that talked about kids not being wanted in certain venues. He wants to enjoy the party and not worry about a child on this particular day it seems. He'd probably love for you to be there with him but knows he doesnt get the full you when you have your daughter. Is it a little immature of him to feel that way? Yep. You obviously arent living together nor are you married, so letting him have his freedom that day as he's asked is about all you can do. You are still in the dating phase and learning about each other. This tells you how he's feeling. He may or may not be as committed as you think or want him to be. When a man takes on a partner that has her own children it takes quite a bit of maturity on his part to handle it. I'd find something else for you and your daughter to do that day, don't pout, and let him have his fun. If the rest of the family are bringing their kids he might find himself wishing he would have made a different decision.
It sounds like your relationship is still pretty new and you've J. begun integrating your child into the relationship.... you definitely have to give this time to bloom. He's not choosing the party over you, he's choosing to celebrate his siblings birthday with his family. You are not quite in the "family" category yet. Be a stand up gal and show your independence from him. Act like it's no big deal. That, my dear, will make him wonder how YOU feel. Wearing your heart on your sleeve too soon is not a good thing. Leaving a little mystery as to how you feel about him is what will keep him interested.