Where I agree that Electrical/Civil/Petroleum/Mechinical Engineers are all very different, you can not say that of Mechanical and Aerospace. Mechanical and Aerospace engineers are kissing cousins.
Mechanical's are a bit more broad in what they study over Aerospace engineers and the differences really come in durring theroy application examples. Ex, in Fluid Dynamic Mechnical engineers will look a fluid flow around spheres, coulmns, down HVAC shafts, and around wings. Aerospace will look at flow around wing planforms and how to improve them.
Aerospace Engineering is really just a specilized way of looking at Mechanical Engineering.
I chose Mechanical to have the broader degree that was more marketable, However I lost enthusiam for it and it was hard to compleate. If he is passionate about airplanes, go for Aerospace as anything that keeps him intrested is good thing.
Missa