Joined: Oct. 2006
People have mentioned some alternatives to aerospace engineering as possible careers. An engineering field that I think will expand considerably in the future is designing and building integrated industrial systems in which the waste from one component is used as the input for the next component. This is already happening to some extent, but the only examples I know of are in developing countries.
Of course, this does not have the same appeal to teenagers as aircraft and rockets, but your son has plenty of time before he needs to make a choice. In the meantime, encourage him in his current interest (as it seems you have been doing). One or two people suggested some text books. At a similar age, and I was good at maths, I would have ignored them. What I did enjoy were books of mathematical puzzles and diversions (I still have a paperback of that name) and books like 'How to lie with statistics', books that did not set out to teach maths but that made use of it.
All sweeping statements are wrong.