You could write "Dear Doctor (last name)," or "Dear Reverend (last name)," or "Dear Reverend Doctor (last name). " You could use his full name instead, but you should not just use his first name.
Reverend Doctors should be addressed with both titles. The title reverend should come first and may be abbreviated or spelled out completely, while the doctor should come immediately next and may be abbreviated or spelled out. If one has abbreviated reverend, one should never spell out the doctor.
