As for NASA...all of the SA's have had astronauts. .
I actually think that USNA has had the most. They had the first American in space, and also the first charged with a felony. Take the good with the bad, right?
I'm kidding...
Seriously, I would have thought USAFA had the most astronauts, but I'm pretty sure it is USNA.
You can also be an astronaut from MIT, or Rice, or a ton of other places (assuming we still have a space program). I think Armstrong went to Purdue for his undergraduate degree.
Sam's point on NROTC vs. USMMA is a good one, the "fun" part notwithstanding. If your son is interested in the Marine Systems major, and he is also interested in commercial shipping (and doesn't mind the compressed academic schedule), USMMA is a good consideration. I think Sam was just trying to point out that USMMA is a big commitment to studying/working in the maritime industry. Naval service certainly can be the ultimate goal, but KP needs to be considered with what is involved in attending. The same can be said of NROTC (i.e. what is given up? what is gained?).
USNA is fundamentally different that USMMA. Thus, if USNA really is the goal, Sam's point on a second application from a civilian university with NROTC is also a good one.
Each person is different, and USMMA might be a better fit, but it's good to know what's involved with each choice.