Hi Almost There
Firstly, I'll be honest and say I know very little about the transition between secondary school to University / TAFE (technical college) and also apprenticeships.
However; being from the generation who as intelligent bright young things left school at 16 I can still remember that major life change transition between school and work (child to adult). It sounds like your son will undergo this gradually in the UK whilst still being at school which is great and will mature/develop him over a period of time learning to deal with adults. My concerns would be taking this young man and then putting him back into a school environment for another 2 years before he can go to Uni / TAFE or an apprenticeship. It does appear that Aussie kids are more mature than the equivilant UK aged kids which might help, but will he resent this additional schooling, does he like the vocational work element of school more than the academic and want to go on to a full time apprenticeship away from a school environment?? So many questions and only your family will know how this will affect him as an individual??
As Rachiegarlo says there appears to be good vocational subjects at high schools in Australia, but I would strongly suggest you researching this and the transition period and also apprenticeships (especially if he will be giving one up in UK by that time). Also 18 months is a long time in terms of his development at this age, so making fixed plans now may not be of much use later as he may not wish to do another 2 years vocational study.
Good luck and my only real advice is that you can't do too much research, as Australia is not the same as the UK in many many respects (this being one of them).