You should explain to him that you are just as capable as he is to take legal action which would put a nice big hole through his funding possibilities. Then, offer to either sign something that says you owe each other nothing and that you are free to do as you wish with the code you wrote. Or, tell him he can pay you to sign the NDA. If your product is good, it's probably not because of the idea. It's probably because of the execution. If he is willing to pay you enough so you can spend 1200 hours on a new product idea. Go for it.