Ok so fyos means first year of school?
And school starts in January, so he either started 5 months ago or will be starting in 8 or so months? I'm assuming the former unless your first year teachers assess pupils well in advance of starting school.
Compulsory school age in Australia is 6 but different states have different starting ages I think, however it's safe to say that he's probably either 5 or 6.
I don't know the Australian curriculum but here this would be the equivalent of a year 1 child working at the level of a year 3 child. Have you taught him to read, given him maths tasks etc, how self taught & motivated is he?
My question to you would be what have the school said they are going to do? If they are doing to differentiate then great, you probably don't need to do much other than follow and support his passions.
In the uk if a child this advanced is at a school which isn't really meeting their needs and they can afford to then parents may send the child to private school as many work about 2 years ahead due to selecting only bright pupils.
Testing is likely to tell you his IQ and his specific strengths and weaknesses. It might show that if his learning were accelerated he could steam even further ahead. However testing is more accurate for children 7+.
If he hasn't started school yet then my thoughgt would be slightly different!