The "Old Boy" is the Devil himself. The Reverend Hale has come to Roanoke to assist Parris in figuring out what has happened in his parish.
Hale has come with an armful of religious texts "weighted with authority." The remark is simulateously pompous and patronizing. To call the Devil the "Old Boy" is pompous because he is not exercising proper respect for the task at hand, should his belief in Satanic powers be real. It is patronizing because it is like a pat on the head to Parris, making him feel lesser because he is not able to handle a problem so obviously solved, given the right person with a proper spiritual education.