Bring a note book and take notes.
Ask him ANY QUESTION YOU CAN THINK OF.
Come prepared with a few to several questions that have been bugging you for a while, or just questions about what ever you want to know more about.
ask him about workflow, reasoning behind solutions to problems, creative reasons why the engineer would choose this over that equipment/effect/etc given context at the moment.
Get his quirks. Try to pick the brain before and after the session if that person has the time, and remember to say thank you!
PS, i'd bring the engineer's favorite beverage as well!