Since you're already on a first-name basis with him, I'd address the letter as follows:
Mr. Rick Shaw
McMahon and Tate Agency
13 Sunset Blvd.
Los Angeles, CA 90121
That is, use his formal name and title in the inside address, but use his first name in the salutation.
Make sure to mention your past correspondence: "I enjoyed our past email conversations, and per your suggestion, am enclosing some samples of my work."