If he was born 2005 I think he will be going into S4 not S3, unless he defers back a year. Not sure if they would even allow that for an October birthday (Scotland cutoff date is end of February and only jan/Feb birthdays can defer automatically) or if he'd want to be in with a much younger class.
In Scotland (most schools), we start doing 9 subjects in S3, then drop 2 to take 7 subjects in more depth in S4. The timetable to S4 normally changes and NAT5 work begins after the Easter holidays. Prelims (you call them mocks), will be December or early January, not really that long after he had started.
It is difficult years to change school systems. Hopefully some of the subject content will be covered in his GCSES, otherwise he will have a lot of work to do.