Dr Ray
You ask which manuals to buy "in order not to just throw 600pound". As an ATPL grounschool instructor, I would suggest that whichever set you choose you will be doing exactly that - throwing away money. The BGS manuals are excellent and will be more than adequate to see your son through his exams.
If you really must buy him such a present, I suggest that you look beyond the training school manuals and toward the textbooks that many use as references. The "Ground Studies for Pilots" series by Blackwell Publishing would be a good start. The UK CAA publish a document called LASORS (available on-line at
http://www.caa.co.uk/docs/33/LASORS_06_WEB.pdf) which inclides somewhere among its many pages a list of recommended reading.
That said, I would second earlier posts - buy him a CRP-5 and a good holiday!