of course this is tough... so many routes to go in this very hypothetical world... but if this were to be, say an interview situation, i would ask him why he always seems so relieved to not "have to" sing. the way he talks about it, it sounds partly out of modesty, but partly out of relief, partly maybe out of disdain. i've always wondered if, when he has been the lead writer on a piece, if he has been surprised at the lyrical direction it ended up taking or if he's even changed the lyrical direction of a song if seemed too incongruous with what he had in mind. or if there were examples where the lyrics were really just perfect.
there are a host of personal questions i wouldn't ever ask and a bunch of personal things i would want to tell him about how wonderful of an effect his music has had on me, and relatedly a bunch of questions i'm sure he wouldn't answer or requests he wouldn't comply with ... but with that all off the tablet, that's the best i can think of right now!