Thank you all for the great input.
Unfortunately the outcome is that my friend and I have decided to part ways on the project, I am taking full ownership of it and any involvement he has will be paid for like a free-lancer. Fortunately I think this happened soon enough that our friendship wasn't ruined over it, and that it will be best in the long run.
You guys were right, having a 50/50 split in decision making was always going to cause problems, and I think deep down I was never really comfortable giving up that much creative control to allow him to contribute properly.
I think we could work together again, this just wasn't the right project to do it on.