Right, according to DH who, as I said up there would like to move into this field but currently works with aircraft, it depends a bit on exactly what he would like to do.
If he wants to be cutting edge forefront of spacecraft design, he thinks pure maths degree and applied maths pHd. Or Physics.
If he's interested in the computer programming side, possibly some kind of computer studies but that's a bit riskier because he would need to go straight from degree into working in aerospace, or he might end up becoming a generic IT guy.
Otherwise aeronautical or mechanical engineering degree with year in industry, or equivalent apprenticeship. If he looks at mechanical the course needs to include two specific units, one is "Computational Fluid Dynamics" and something that covers"Jet Propulsion, Rocket Propulsion, Ballistics".
For now he could look for a STEM club in his local area/school. Model rocket amateur clubs in his local area. If they're local you've probably been to these places already, but the space center in Leicester and science museum in London are great in general and have special events if you keep an eye on their websites. This is true for trying to get on any degree course but if you can show you're taking your interest in a subject further in your own time that's always going to help your application.
Needs maths and physics at A-level definitely, Chemistry also potential a good option (I'm sure you've thought to look up the potential university courses and check what qualifications they require).
Hope that helps.