In an interview, how should I handle the question asked of my construction salary requirements?

I am a construction recruiter and I have a “great “ construction executive for a position. However, I can’t get this construction executive to tell me what his construction salary expectations are based on his history. Sounds like this construction executive is having a hard time with trusting you. First, restate that construction salary is needed to determine his eligibility for the position. Give him an idea of what you construction employer pays employees in the same position. Reiterate that construction salary is strictly confidential but required in order for both of you to prescreen each other. You may also explain that construction salary is based on experience and knowledge and not on his prior history. He could be withholding due to insignificant pay as to what he feels that he is worth or he may not want to shot himself out of the ballpark.