I just lost a longish post I'd been typing, but others have said some of the things I'd been thinking in the meantime!
I think the biggest issue to consider is university (or alternatives) - what he might want to do, where he might want to do it (would he ask to stay in Australia for example?). There may be a few specialised situations where you can get home fees after less than 3 years living in the UK, but they probably won't apply to you (check!).
After thinking through that, work backwards as to the routes your son might take. I know of someone who did home-schooled IGCSEs in NZ prior to UK 6th form, that possibly made it harder for her university applications (I think she was initially applying for medicine though). Definitely do some research together about what he might need for applications.
If currently in year 10, does that mean you'd have 9 months "up your sleeve" for him to prepare for 6th form study? I think he'd need to make the most of that. IB courses are available at some private schools - could that be an option that would make sense?
Non-university higher education options are probably quite different in UK and Australia, if that's a relevant pathway again worth looking at carefully for decision making.
Don't forget that the Scottish system is different if you might end up there.
Good luck.