I'd ask him what his greatest problem that persists to this day is. The proverbial "pebble in his shoe", "thorn in his side", "splinter in his mind" -- the greatest engineering problem that he has -- that continues to persist to this day.

If he doesn't have one, then guess what? He's not all that senior because he's not looking for such problems -- he's not challenging himself, he's not looking for his next step in turning something previously thought impossible -- into something conceivably possible. (I wouldn't tell him that in the interview though, for obvious reasons).

You could ask what his greatest engineering challenge was historically, or what he thinks would be a really tough engineering problem to solve in the future, and go from there.

Either of those questions would be good avenues of inquiry and conversation...