Hello there
If your son is starting an integrated course he should really be getting the manuals free as part of the course. I am currently a student at Oxford so am biased, i do think the manuals are really informative, however, i wouldn't recommend reading them without the top class tuition that one recieves from the instructors, especially here at Oxford! In fact it can be quite daunting when you first see the manuals!
I would recommend revising maths and physics. Oh yeah and for him to enjoy himself before he embarks on his course, because it's going to be a tough course, especially the groundschool and unfortunately the only socialising he'll be doing is with his atpl manuals.
Hope this helps
Bozzato