A good intro is J.F.C. Fuller's "The Generalship of Alexander the Great", as well as Plutarch's essay on Alexander. Great as intros, but somewhat idealized portraits of Alexander.

The definitive book in my mind would be Peter Green's "Alexander of Macedon". This is the most balanced and scholarly writing on Alexander I have ever read, by an expert on the Hellenistic Age.

The Landmark series has a beautiful edition of Arrian's "The Campaigns of Alexander". This is one of the most reliable source texts we have on Alexander, written by a Roman who allegedly had access to the journals of a man close to Alexander.

These are more historically reliable to my knowledge than what was seen in the movie which was based on the work of Robin Lane Fox. I think Fox buys into later Roman sensationalism and lacks prudence in his work (reporting things that likely didn't happen as fact) but that is my opinion. I rolled my eyes a lot at the movie.

As a primer on Greek history, he might also consider starting at the beginning, with "The Histories" by Herodotus. I recommend The Landmark series again because of the maps, but the penguin edition is pretty good and has a handy outline in the back.

Next would be Thucydides, also available in The Landmark series.

These last two give a decent picture of the Greek world just before Alexander.

Good luck. It is a fascinating story.

[edit: clarification]

How old is your son?

I did a subject at uni the year before last and we used Pierre Briant's "Alexander the Great and His Empire". Lots of maps and straightforward language.

Semi related question, how reliable of a source is Arian? I read a translation a few years ago, and I've been wondering ever since whether I can really use it as a source at all.