Some consider him to be in the same league as Einstein and Newton. Meeting him for lunch -- just a random opportunity that I got due to a common friend. I want to make the best use of this time -- so sourcing ideas on what I should ask him? I will post his responses back here.
Seriously, after reading his wiki page, he obviously seems like one hell of an intelligent person. But something else struck me, he's been married since 1979 to the same person who is also a professor of physics, plus they have kids that all appear to have done quite well. I'd be more curious to see how they balanced their life and were successful in raising kids that have passion and are doing well, at least from their basic bios.
Everyone probably asks him physics questions, but chatting with him about the everyday things to me could be way more interesting, and wouldn't feel interviewee. But that may just be me. I would want to ask him what he feels is his best accomplishment professionally and personally. That is always interesting to hear.
Ask him about his interests beside physics. Philosophy. Art. Whatever.
But most of all, don't pester him. If he seems receptive to those "big questions", great! If not, just do normal smalltalk.
Don't worry about it -- be yourself.