The experience of living abroad and having a 'different' school experience is bound to enrich his life in other ways. If you are concerned about academic standards (and due to later starting age and curriculum, I would say that Uk schools in general are ahead of NSW ones) you can always find loads on online help and give him extra support in the 6 months leading up to your return to the UK (if you do actually decide to leave Australia at all!)
There is now a national curriculum. It came into play two years ago as that's when prep became compulsory in qld. Also the ages must be different in vic? In qld its school (prep) the year you turn 5 by July. No exceptions unless a paed of special ed system says. Ds has special needs and still started this year and just turned 5 in April. He will just repeat prep.
It is confusing with the different names. There isn't yet a curriculum here so schools do vary. I taught a mixed kindergarten/grade one class in Sydney and it was nothing like reception. No dressing up corner or sand trays. Iam a supply teacher in Melbourne now and the schools here vary hugely. Some are terrible and have hardly any resources.
If you want to make sure your ds isn't behind when he goes back, I strongly recommend that you go to look at schools with that in mind. Things are far more laid back here so you are going to need to find a school that places a high value on academic achievement. If that's what you want. Some schools place an emphasis on other things. Our is all about the environment and we have a playground set in bushland. You can see huge eucalyptus trees from every classroom window. We have an orchard and a pond. My oldest is right at the very top of her class but shes behind compared to the UK year 4. She can climb a tree though!
So don't be swayed by art rooms, a massive school play a year, pet goats and no reading books at all like we were
Regarding ages too, children can start school so long as they turn 5 by 31 July of Kindergarten. However, the confusing thing about NSW is that is they turn 5 anytime between the Feb and July of their first year (kindy) you can choose to either send them or hold them back for the following year to start when they have already turned 5. There is huge debate about this as there is a current trend to hold back younger children (particularly boys) with the idea they will somehow be leaders or better at sport if they are the older ones in the cohort rather than the younger ones. However, you don't have to hold back and so long as your child turns 5 by 31 July they can start (at a public - non fee paying govt school) although some private schools will have an April or May cutoff date. Confused now??
Sorry - vic naming is different to NSW (I think the substance of what they learn is the same). . I can't compare us to the British system but I can tell you the following (my kids are at school in NSW)
Preschool is the non compulsory 1-2 years you can do before formal school (eg 3 year old and 4 year old pre school). No reading or writing, mostly play based learning and used for socialisation prior to formal school. Often half days. Childcare centres run preschool programs as do some private preschools and public funded preschools.
Kindergarten is the first year of formal school. Children start learning to read and write in kindergarten. It is compulsory. After this comes years 1, 2 etc. kindergarten is found within the primary school.
Within each of the states, there are different names used for the early stages, hence the confusion above.
Hi - we're based in Victoria, not NSW, but I think it's similar up there to here (I know it's similar in QLD) so I'll try to help.
Kindie is preschool/nursery - children can do a year of '3 year old kindie' (the majority will be 3 before the start of their 3 year old kindie year) and a year of '4 year old kindie' (again, most kids will be 4 before the start of their 4 year old kindie year because of the way the school entry cut offs work). Kindie isn't compulsory and it's not actually 'school' but a preschool/nursery program (so my understanding is there's a lot of focus on interacting with others, creative play, communication, building confidence and some early numeracy and literacy, rather than sitting down and learning how to read and write; basically getting ready for 'proper' school).
Prep is equivalent to reception year in the UK. It is the first year of primary school. Children begin to learn to read and write then (though my understanding is that they will have begun some basic work on literacy and numeracy skill at kindie).
Year 1 follows prep, just like year 1 follows reception in the UK.
My impression is that children are generally older here when they start prep than they are when they start reception in the UK. My DS, for example, was born in July 2010. In the UK he was due to start reception in September 2014, 4 years and 2 months old, but here he will start prep in January 2016, at 5 and a half years old. So if we moved back to the UK in October 2015, for example, our DS would be meant to be in year 1 in the UK already but wouldn't even have started prep here yet. (Unlikely to be a problem for us as I'm from Australia and we've moved back permanently, but something you might need to think about).