Assistant Surgeons were Captains and Surgeons were Majors. The Surgeon General was a Colonel. NO Lt. Colonels in the Medical Corps! Medical Directors/Corps, Division, and Brigade Surgeons were all Majors, their assignments notwithstanding, and seniority was based on the billet.
The William Bonner you have was old enough to serve as an Assistant Surgeon! Are you sure the DOB of 1848 is correct? I think you have the right man, but wrong DOB!