I have to agree with Scroggs and the other posters
DON'T BUY MULTIPLE ATPL NOTES
If he's signed up with Bristol GS, the notes he will get along with the tuition will see him good. He / You have made a good choice there. I did all 14 exams with them, first time passes in all. An excellent school in my opinion.
Please don't take this the wrong way, but neither you or your son have experience of the ATPL theory process and when those that have tell you not to buy multiple sets, they do so from experience. It's merely duplication and they will say the same things, albeit from a slightly diferent perspective.
This game is expensive enough as it is. I would put the money towards a CRP5, online question bank from BGS, accom and food on the crammer and any left over to go on the flying afterwards.
Good luck to your son. It's hard work, much harder than a degree in my opinion (Mech Eng at least !!!)