Well, for starters, it would probably be a good idea to look at what his lab actually does. Assuming you're actually interested in his field of research, you could see if you've used any techniques that might be applicable to working in his lab, or if you've taken any relevant courses in that subfield. Good questions to ask might be something along the lines of "What are your graduate students currently working on?", "How many papers do you students publish during their time in the lab?" (that one can vary significantly depending on the field though), or "What kind of projects are you planning on working on in the next few years?". You want to interview him about his lab at least as much as he wants to interview you.
You don't need to act like an expert in his field - its unlikely he expects you to know all that much about it at all, unless you've worked in the field previously (in which case you should know at least a little about the field). Don't sweat it.
The last thing is, don't act like you're not qualified (even if that's how you feel). That's his decision to make, not yours. If he's bothering to interview you, it's because he thinks you have some potential, or he wouldn't waste his time.