<p>Sorry if I came off as jumpy or whatever. Applied math or physics would be very versatile and would be acceptable to nearly any graduate area of study in engineering, as far as I can tell (like others have said, a 1-year Masters might turn into 1.5 or 2 year masters, but in the long run its not that big of a deal). From what I've seen, undergraduate engineering degrees are acceptable for acceptance into most quantitative areas of study(economics, physics, math, CS), but (at least from the postgraduate survey from Cornell) you won't see people going from engineering to, say, classics. I think that was your original question.</p>
<p>If he did what you are suggesting (applied math or physics), then he could even probably take electives in mechanical engineering his junior and senior years if he decided on aerospace engineering as a career. So he could basically decide at any time what it was that he loved/wanted to do and go ahead and start pursuing it (still getting the BA/BS in Physics/Applied Math) through electives.</p>