Seriously, he really needs to think that aspect through.
Ern made a very pertinent point and so I'm repeating it. 24 is not too late at all but it's a bit out of cohort.
A couple of observations from a self-admittedly biased ex-Naval Officer - first thing he has to decide is that he can put up with being in the military, period, and only then which branch. All of them require more personal discipline than most civilian careers and make demands on your personal time, your location, your ability to interact with family, not to mention appearance and diet. If any of these is a show-stopper it's good to know it before he takes the oath.
I am the son of a career Army officer and so I know that end of it as well. My late Dad was, at various points in his career, a buck Private in the horse cavalry, infantry, Signal Corps, and Corps of Engineering. The point is that there is opportunity within each branch to find something you're good at. But it's going to be driven by the need of the service at the time. I did not actually expect to end up driving ships myself but that's where the need was and so that's where I went. It's like that.
Best to you and to him.