I'm in exactly the same boat as you, although double-the-trouble as we've got twins who turned 4 in April of this year.
I'm in Darwin now, with the family following in a few weeks.
My understanding is that, as you say, our kids would be starting school in Sept in the UK, but over here they'll be starting when the kids go back to school at the end of January next year.
We've already managed to reserve places for our older kids in primary & secondary schools, and there is a pre-school attached to the primary school we've selected.
Depending on where you're going to live, perhaps there would be a similar pre-school attached to your local primary school and your son could start there on arrival and then move up to the primary school in January?
Hope that helps.