1) Is hours building now the best use of his time ( apart from working and saving up lots) while he waits to be old enough to commence the next stage ?
As has been said he can hold a CPL at the age of 18 but he would need to have 150hours before starting the flying side and most schools insist on have the theory done before the flying - that saves a student problems if they struggle with the exams!
You have three years from completing the ATPL theory to finish all the flying exams so a bit of time
2) Is it worth him doing any of the other exams as soon as he has the PPL or is this better awaiting until he moves further along the process?
See answer to 1)
3) He wants to go out to the states when he is 18 at the end of his a levels for a break and was his ppl obtained in the uk allowing him to do this. Could someone please advise what he will need to have in order to get himself some hours there. He was looking at both Florida and Arizona.
Hour building int he US if fun and if done correctly, rewarding and profitable but not without its pitfalls. Firstly you need either a “piggy back” licence off your existing PPL or to complete a stand-alone PPL. You’ll need TSA approval prior to undertaking any training/test in the US and usually this is quite quick but can, on occasion, drag on.
Be careful that any training done in the US is logged appropriately as some EASA (and the U.K. used to be an offender) disagree with the FAA on their definition of P1 hours.