Hospital Corps School (A School) is only the beginning in a U.S. Navy Hospital Corpsman's career.  Nearly all will attend the eight-week Field Medical Training Battalion program, either directly after A school or C school, or following a duty station posting.  During A school, corpsmen will have the option of selecting a C school and therefore a specialty, i.e. surgical technician, to attend right after A school.  All this is determined by a phrase you'll come to hear throughout the Navy career:  "It depends on the needs of the Navy."  During A school they meet with career counselors and submit a C-school/duty station 'wish list', but there are no guarantees.

Wikipedia has an excellent page describing the HM rate that's worth reading all the way through; here's the link:  U.S. Navy Hospital Corpsman (HM) - share it with family and friends, too so they can understand what your sailor is embarking upon. To paraphrase -

A Hospital Corpsman is an enlisted medical specialist in the U.S. Navy who serves with both Navy and Marine Corps units.

The basic training for Hospital Corpsmen is conducted at the Medical Education and Training Campus one of the Navy's "A" schools (primary rating training), located at the Joint Base, Ft. Sam Houston, TX.  Hospital Corps School is also known as Basic Medical Technician Corpsman Program, or BMTCP, in conjunction with the Air force.

Most corpsmen will go on to the eight-week course Field Medical Training Battalion (FMTB, or Field Med) offered at Camp Pendleton, CA (FMTB-West) and Camp Johnson at Camp Lejeune, NC (FMTB-East).  At Field Med, they receive advanced emergency medical and small arms training, and learn basic battlefield tactics while becoming familiar with the basics of Marine Corps life.  Corpsmen do not have a choice in location, by the way :)  Detailed description and much more here:  http://www.tecom.marines.mil/fmtbn-e/Students/Welcome/FMST.aspx, click on "Course Description."

"Blueside" corpsmen serve in a variety of capacities and locales, including on ships and in hospitals.

"Greenside" corpsmen serve with U.S. Marine Corps operating forces. 

FMF (Fleet Marine Force) qualification is earned by Navy enlisted members assigned to the Fleet Marine Force of the U.S. Marine Corps who have successfully completed the necessary requirements of the Enlisted Fleet Marine Force Warfare Specialist (EFMFWS) Program. This involves serving with a Marine Corps Unit, passing a written test, demonstrating skills used in service with the Marines, and an oral examination by senior enlisted sailors who are FMF qualified.  Enlisted Warfare Specialist FMF Device Program: http://www.lejeune.usmc.mil/2dfssg/med/FAQ.html

The rating of Hospital Corpsman is the most decorated in the United States Navy with 22 Medals of Honor, 174 Navy Crosses, 31 Distinguished Service Medals, 946 Silver Stars, and 1,582 Bronze Stars.  Twenty naval ships have been named after hospital corpsmen.

CORPSMAN PLEDGE

I solemnly pledge myself before God and these witnesses to practice faithfully all of my duties as a member of the Hospital Corps.

I hold the care of the sick and injured to be a privilege and a sacred trust, and will assist the medical officer with loyalty and honesty.

I will not knowingly permit harm to come to any patient.

I will not partake nor administer any unauthorized medication. 

 I will hold all personal matters pertaining to the private lives of patients in strict confidence.

I dedicate my heart, mind and strength to the work before me.  

I shall do all within my power to show in myself an example of all that is honorable and good throughout my Naval career.

Annual Hospital Corpsman Ball celebrating the birthday of the rate June 17, 1898.