So.. weird thing.
Michael mentions being a Corpsman when setting Harry's arm.
However in Grave Peril (I think) he mentions that only Charity has medical training.
Now while a Corpsman wouldnt say they were a doctor. They do have medical training -thoughts?
From what I recall of the last time this came up was that it actually fit pretty well. The crux was the relative levels of training and practical use of a medical Corpsman versus something more like an unlicensed Registered Nurse. The sort of training Michael would have received would have been more akin to an EMT/paramedic, a sort of Extremely Advanced First Aid training, but being a single-tour recruit he'd have been the /help/ for the true Doctors for anything more. Michael's knowledge would have been focused on basic diagnosis and procedure-based treatments, but they'd need to make him useful quickly. By contrast Charity could receive a great deal of academic medical training at home via various courses to be a Nurse Practitioner. If she's motivated enough to learn Armor-smithing and sword-fighting to support Michael, then devoting energy to Medical.
*Note that Im regurgitating what I recall of the topic, I personally have no first-hand knowledge of either the military or the medical industry.