I was on a similar position some time ago. I do think your co-founder isn't ready for the startup ride. You can talk all you want with him, but at the end it

I was on a similar position some time ago. I do think your co-founder isn't ready for the startup ride. You can talk all you want with him, but at the end it will come back and bite you. If he isn't passionated about it now, he probably won't be in some months. If you really need to start rolling, go ahead, talk with him and tell him (gently) you're going on your own. As many others have said, trust your gut feeling! Generally, the first reaction is the most visceral and truthful one. So if he is reacting like this to the incorporating issue, don't fool yourself, he's not the right partner for this venture. Good luck