Look for something that will turn into a good yarn - ask him what the naughtiest thing he ever did was - or the most trouble he ever got into. Could be quite funny, but it can reveal a lot about how things used to work too.
When my granddad was a kid he used to have a job cleaning out and lighting the fryers in the local chippy before he went to school, back when the fryers were coal/wood fired. Anyway, he was arsing about and inadvertently burned the chipshop down to the ground.
The whole thing ended up going to court, the chippy wasn't insured (and I guess in those days you wouldn't expect it to be) and my great grandmother was deemed to be liable for her sons actions, and of course she wouldn't have had public liability insurance, so to compensate the chipshop owner she was sentenced to cook the guy his supper for the rest of his life.
If you are doing recordings you'll find that its worth doing a few of them over a period of time, partly to get him comfortable with being recorded, but also stories are better than answers to questions, so its worth revising anecdotes as they'll be likely to told better second time round, but you might also find new tangents and diversions when you go over old ground. You want to arrive at a point where you hardly say anything and the stories tell themselves.