I went through it myself... and more or less at the same age I arrived in Australia in March having completed close to half of 7th Grade in the US (what was then known here as Form 1)
While it was half a lifetime ago, I would assume what happened with me, is more or less what would still happen today.
Ultimately what will happen is your child will end up skipping half a year or repeating half a year. But the reality of it is more nuanced than that because academic programs (ie what is taught when) will vary between the two school systems. whether they skip or repeat, there will be material that they they have already learned which gets repeated for them, and there will be stuff that gets missed because it wasn't taught yet in the US, but has already been taught here.
My school had me sit a general test that covered general aptitude across a range of disciplines relevant for a Form 1 student. Social fit came into play here too... size, maturity, etc.
For subjects like Maths, I was a bit behind at the start, but being academically minded, I caught up pretty quickly. Basically I "placed" differently on each topic... History.. (strangely Australian history wasn't taught in school in the US but was about on par on anything other than Oz.), Enlish well advanced, but I came from an academically minded family.. and so on...
In my case, I skipped half a year. Funnily enough it wasn't until University that skipping half a year it made any difference.. due to when during the year I was born, and I was effectively a year younger than my cohort. Under age for drinking, driving etc as a first year student was far more of a cramp on my lifestyle and experiences than taking longer to get a grip on Algebra topics that I missed..
Given transition from Primary to Secondary schooling is a huge transition in of learning styles and curricula, social groups and the like, if you land, and he drops directly into 7th grade for the start of the school year, I doubt he will notice it much even if he effectively missed half of 6th grade.
In Victoria at least, Melbourne was in lockdown for a significant part of the 2020 academic year and the Dept Education put a lot of materials online. Every State does thing differently here, but it will give you an idea of what is being taught here
Resources to Victorian government school staff.
www.education.vic.gov.au
NSW Dept Education may have similar..
At the NSW Department of Education we educate and inspire lifelong learners – from early childhood, through schooling to vocational education and training. We ensure young children get the best start in life by ing and regulating the early childhood education and care sector. We unlock...
education.nsw.gov.au