I am corresponding via email with an industry researcher regarding an internship. He always signs off his emails with his first name, Bill. Still, I have addressed him as "Dr. X" for several emails now. I realize that if I am offered this internship I wouldn't want to keep calling him this and would prefer to be on more casual terms. Should I just abruptly switch to his first name, or wait for him to actually tell me to use his first name?