I did informational interviews when I was looking to move up in my job. I scheduled them with a couple of people in that role, and just met them in their office for a half hour or so. I had specific questions about the role, so that was what most of the conversation entailed. What they liked most and least, what a typical day was like, if they thought I would be good for that type of position. Yours would obviously be more general about your field.
Do you attend anything where he might be to set up a discussion? Have you connected with him on linkedin or joined any professional associations where you can speak to him? That might be a place to start.
My gut is saying it would be fine to e-mail him, re-introduce yourself, and tell him you're looking to move up in your career. He's an expert in the field, and you wanted to meet for coffee to talk about the future in that field and how you can be a part of it in the best way possible. He'll probably be flattered. I'd definitely have questions for him, and expect it to take a half hour to an hour.
My only concern is that you not structure it to look in any way like a conflict of interest with your current job.