Have you done this specific type of project before?
What would be the client contact?
If an architect, was involved what is his contact information? (Architects can advise of many aspects of the process that may not be known by the Owner interms of substitutions, performance, quality compared to competitiors, etc.)
Will YOU (the person being interviewed) be the one running the project from day to day? Is not, could I meet and interview that person?
Ask whether the subs he is planning to use on your project are the same ones he used on the project he is using as a reference. (If not, you may well get a different result).
Ask concerning any BBB complaints (always check out BEFORE meeting him).