I'd ask him why he created the earth at least three days before the rest of the universe, and how he managed to separate light from dark on the first day, and have evenings and mornings on the first three days, when the sun and the other stars weren't created until the fourth day. Wouldn't it have been better, I would ask, to have invented the sun and the rest of the universe first?
(I shan't let on, of course, that we know that the latter is what actually happened; no sense in upsetting the old fellow with our superior knowledge.)
I'll also ask him why there are two mutually-contradictory accounts of his own son's birth, or hasn't he read the OT yet?