Ask about the problem children and squeaky wheels (regarding to servers), that will get you down to the one-off fixes that are held up by bits of string and expect scripts that rely on chaining ssh across 5 machines to touch a file that doesn't exist. Ask about the oldest equipment, and spend some extra time getting to know your world. Leverage the time you have with him, when he goes home for the day scour the network and start looking at boxes and going over notes, when you run into problems write them down and ask him about those.
And you dont have a week you have 4 days, because the last day he may show up but you most likely wont get much out of him unless its a quick Q and A. That and keep your resume up to date because you may want to use it again soon.