A good way to start could be drumming up a short list of concise, but interesting questions you might ask him about his work. Then try to connect it to your own work or tease out how your interests going forward actually build on this idea. You can try to ask if he/she'd be interested in collaborating, but big names are notoriously hard to get a hold of, even for their own students, let alone other students outside their wing. Don't know if this is the case for your field, but I'm fairly sure it's a common problem.
A more realistic expectation at this point would be for you to (1) pique his/her interest and ask if it's alright to keep them updated about your project or keep in touch with his/her work. (2) Begin the project on your own, and keep them updated when you've demonstrated a good amount of progress (and success!) in the project. (3) Then, down the road, they'd be more interested/motivated to collaborate on a paper.