I really wish you the best of luck. I am so glad you are taking a stand and allowing your son to experience a birthday party. Hopefully he will go to many more and even have his own.
I did the same with my kids. You will be responsible for all planning, gift buying and wrapping and transportation. It's worth it! The bond you have with your son will grow stronger as you do these unapproved things and mom isn't around. my wife and I argued the first couple birthday parties but eventually she softened and although for the most part she has nothing to do with it she has given our kids a ride or two when I was sick. I even think there was time when she had to pick up the gift at the store for us for a bday party.