Great suggestions, DarkSociologist! Good stuff. For some reason Iry you have ticked this guy off. Lunch seems a great idea. Possibly there's no point going into the nitty gritty detailing stuff straight off and 'who did what and when' talk, but to focus on how you agree how to work together to start with. get whatever grievances out of the way. Obviously he's not a very happy chappy at the moment and if you want to continue working with him it's time to let the man speak out. Think about what he says before reacting. As it's teamwork the two of you need to work it out, but now wrestle it out
Good luck