More advice: if you do part ways, I woudn't worry about or do any fighting over him "taking the idea". In all honesty, despite what the USPTO might have you think, nobody can own an idea. Lots and lots and lots of people have been independently coming up with about the same exact ideas for centuries. The trick is execution, and timing, and luck, and polish, and firing on all (or enough anyway) cylinders. I have dozens of notebooks full of ideas --- sitting on a shelf, and therefore worthless. There's a very very high likelihood that there is somebody already out there executing on the same idea as you are now. They might be a little ahead or behind, or a little different in various ways, and you will never escape that contextual reality. The trick is what you do, and what you make of things.

Fighting over ownership of an idea will just lead to more grief for you, for him, and for the community -- except for lawyers, because they'll love the billable hours thank you very much!