Looks like your son and I are on the same boat. I too wanted to be a Corpsman, but got Machinist Mate and also volunteered for Submarines. A better choice in my opinion.
Here's a video of the
Silent Strike Force which I found very informative. Another link,
Basic Enlisted Submarine School, which is a 26 day course. Have a look at that so he knows what to expect at BESS.
And lastly, a
Submarine FAQ, which has a ton of information about submarines. Unfortunately, I don't know much about the A School other thank its location, GL, and that it's 4 weeks long. Nearly forgot about the Dolphins. Go to this link,
Submarine Warfare Qualification. Every bubblehead has to go through that process to earn their "Dolphins" pin.