Couldn't help but join to comment on this thread.... (for background I'm currently writing up for a PhD in astronomy)
For A levels, as everybody else has said Physics and Maths are essential. Further Maths is good, but more from the 'it'll make the first year of degree easier' than being a course requirement as almost all Physics/ Maths courses will cover the FM material in the first term or so of teaching to ensure everybody is at the same level, whether they did just maths, fm, IB maths etc. So if you've done FM its good since there'll be more topics you're familiar with but you go into more detail with them than you would at A level anyway.
As a fourth option, I'd go for chemistry - again there's a fair bit of overlap with some first year physics stuff that A level chemistry - at least the inorganic stuff - gives a nice bit of context to. I have yet to meet an admissions tutor who's been that bothered about the student being 'rounded' at A level and studying a humanity/ arts subject as well.... if you've got physics and maths its not going to count against you, but its not really going to help either IYSWIM?
Reading around the subject is great, but it should be something he should do because he's interested in it, not because he thinks he should for uni! As others have said New Scientist subs. is a great way to keep on top of developments and get a 'feel' for the subject. Bookwise, I would personally avoid 'A brief history of time' (some of its chapters are somewhat out of date, plus its not written in the most accessible style) but you may want to try:
- 'Black Holes, Wormholes and Time Machines' by Jim Al-Khalili. I LOVE this book. It's aimed at a teenager audience, and its what got me into physics; I picked it up when I was 12 and thought 'wow, this is so cool!' (I was/am a complete nerd though....)
- anything by John Gribbin, he's written tonnes of books on a whole range of astro and physics topics
- anything by Paul Davies, ditto to JG
- anything by Ian Stewart, he's written a lot of good, accessible books on maths; I particularly liked 'Flatterlands'
He may also like 'Astronomy Cast' (downloadable iTunes) - I'd rate this as the best astro podcast available - and may like to have google NASA's 'apod' (Astronomy Picture Of the Day), as it gives a nice idea of the scope of the subject (plus they're just cool!). TV wise, most of Jim Al-Khalili's programs are pretty good, as are Brian Cox's on particle physics/ CERN. I'd take the 'Wonders' series however with a pinch of salt, they are admittedly pretty to look at, but frankly content wise they are less than impressive (I have yet to meet an 'actual' astronomer - BC is a particle physicist and it shows - that liked either series).
As others had said, see if you have a local amateur astronomy group that run a youth section, and if you live close to any Unis, see what outreach events their physics/astro departments run. Most will run some sort of public oriented events, whether yearly open day or weekly public talks. If you're near London, then the Ri (Royal Institution) run public lectures all year round, not just at Xmas! Further south, the Intech science centre (near Winchester) have a planetarium and run excellent series of talks. Further north, in Leicester there's the National Space Centre too.