Firstly, well done on reaching the partner round!
Partner round interviews are going to be more about how well you fit within the firm's work and social culture. To answer your question, you can be very flexible in deciding what to ask him of course - you don't want to sound like every other candidate by asking him roted questions. But keep in mind the following while framing the same:
1. How long has he been at the firm? This will help you understand if he has seen the firm evolve over time or even revolutionalise in the different market eras (eg. the current digital). You can then ask related questions.
2. Is he an operating or a selling partner? This can help you quickly understand his day-to-day activities. You can then ask questions regarding this.
3. Does he operate within a specific sector (retail, tourism, healthcare) or area (operations, marketing, procurement)? If any of this relates to your profile, good for you! Strike up a discussion.
4. Values of the firm are important and understanding how much a partner encourages their prevelance in the firm is even more. Ask a question that helps you understand the firm's work culture - what are the short term goals they are working towards? Is there a Vision 2020 they have? How does he think they're doing on it and what remains to be done?
Yes, as mentioned, please do not ask obvious questions which can be found on the firm website. Do your homework as the partner has to feel that you're genuinely interested in joining the firm. These are senior people with long years of client, business and interview experience. With them, it's all about making the interview hour a pleasant break from their busy schedule.
Little tip: Make the questions personal and not generic eg. his roles, his ideas, his experiences.
Good luck with your preparation and interview!