Wankersetc - the OP mentioned she'd recently moved from England, so I'm assuming she's new to the Scottish education system.
I agree that it's worth ringing the school: there will be some teachers in early next week with the results coming out, plus all teachers will be back from next Thursday for their Inset Days (prior to the pupils starting on 15th - eek!
)
They should have experience at how to cope with new kids arriving in from other system (although maybe ds' school, with so many students from around the world, is more used to it). A report from the old school will help, at least to assess where to put your ds in those subjects that are set. They may put him in classes and then adjust if necessary after the October Week.
Technically the Nat 5 curriculum is over 1 year but as Wankersetc says, many of the schools that do a high number do it over 2 years. Shouldn't be too much of an issue for you/your ds as they're going to have to assess what his "base" is anyway.
I'd suggest concentrating on the 5 subjects that you think he might want to take on to Highers, to make sure he does well in them. The rest are optional extras.
FWIW - many many
years ago, I arrived back in Scotland 6 months before the O Grades that dates me from another education system (NZ) and had to "catch up" in all my subjects (and learn Latin from scratch
).
The difference was that I was returning to my old school after a 2 year gap (and a tantrum from db and me when my parents bought a house in the neighboroughing suburb and it looked like we might have to go to yet another new school), so they knew what I was capable of just as well as Dad managed to forget to pack my brilliant school report 
I did end up studying Hamlet for 3 years in a row as my class was half way through Macbeth and I'd studied Hamlet in NZ and then I studied Hamlet again in S5 for my Higher. It just meant I was slightly restricted in my choice of questions at O Grade, as I had to answer a generic Shakespeare question as opposed to a specific Macbeth one.
You might find that the emphasis in Maths (on particular), Physics and Chemistry is more on applying the knowledge to show understanding rather than rote learning of formulae as this is a key part of Curriculum for Excellence.