I have unofficially mentored a dozen entrepreneurs over almost half a decade and the only thing I can tell you is the exercise is a waste of your time.
The feedback driven growth model taught in your favourite b-school does not work on entrepreneurs. The best entrepreneurs are the lousiest on taking advice.
I did figure out what works after a long time, but it is completely counter-intuitive. You can do two things, and two things only:
1. Give the entrepreneur confidence. This is in some ways the exact opposite of honest feedback.
2. Once you have given him confidence, try to extend 'his' thought process based on your knowledge/experience. It is too hard to explain what I mean by this and I won't even try. I am unable to do this in most of the cases.
"Unfortunately, no one can be told what the Matrix is. You have to see it for yourself. "