You might want to look on wikipedia for navy corpsmen.
In the Navy, you go in as an E-1 -- recruit. That is your rank at Great Lakes, which is the Navy's recruit training. If you do your pre-enlistment stuff and complete it and demonstrate that you know it during training, you get promoted to E-2 -- apprentice. Training officers (Petty Officers not officers) can promote up to 10% of each class one grade, which means that you can come out of recruit training as an E-3 -- seaman or airman. This is your maximum advancement through recruit training.
If you're doing good, and have the time in service (I think 1 year, but I'm not sure), you then go for E-4. You need to take an exam of which half the test is your job skills and the other half is general Navy knowledge. An E-4 is a Petty Officer third class or the equal of a corporal. Again, after sufficient time in rank, you can go for E-5, Petty Officer second class, or the equal of a sergeant.
After a longer time in service, you can go for E-6, Petty Officer first class. After that (E-7 and above), you are a Chief Petty Officer (commonly just called a Chief) and have significant responsibilities.
Beginning ranks can be skipped if you have significant professional skills prior to the Navy. In other words, if he was a lab technician, he could go out of recruit training as an E-4 or E-5. No prior training and the Navy doesn't care how smart you are, you don't skip ranks. Though if you're really smart, they'll want you to go nuke.
Ranks for corpsmen are as follows:
- HR - Hospitalman Recruit (E-1)
- HA - Hospitalman Apprentice (E-2)
- HN - Hospitalman (E-3)
- HM3 - Hospital Corpsman Third Class (E-4)
- HM2 - Hospital Corpsman Second Class (E-5)
- HM1 - Hospital Corpsman First Class (E-6)
- HMC - Chief Hospital Corpsman (E-7)
- HMCS - Senior Chief Hospital Corpsman (E-8)
- HMCM - Master Chief Hospital Corpsman (E-9)
As your rank goes up, your training and skills go up. A hospitalman is probably just changing bedpans. Most of the semi-skilled positions (lab tech, x-ray tech, so on and so forth) are going to be E-4 through E-6.
Nurses are officers. I don't know what rank they start out at.
Marine corpsmen (all Navy) have specialized training, as do paramedics.
If you know what actual position you want your corpsman to be, talking to your local recruiter would enable you to find out the specific training program he would have followed. The military involves numerous schools as your job skills and rank improve.
For example, my daughter, who's a Petty Officer third class (and likely to get a promotion in December) is presently learning to be a paralegal. This program is not available to seamen. You have to be E-4 and above to take it. If she becomes a Chief (several years down the road) she'll have to go to a school to teach her how to run a law office, as well as some other schools.
Hope this helps you.
Best of luck,
Jim Clark-Dawe