Australian seats are not designed for extended rear facing and should not be used past the recommended weight limit. The Swedish seats, which do support extended rear facing, are not approved for use in Australia because they use a different tethering system and Australian standards require all seats to be bolt tethered. Many European seats (including ERF seats) use the Isofix latching system which is actually safer, ironically enough. This system is available in many cars sold in Australia, particularly those of European make, but cannot be legally used.

Don't get me started...

Short answer is, no, you cannot legally rear-face your child once he/she reaches twelve kg in weight.

However, if you can get a doctor to authorise the use of a non-standard ERF seat under medical grounds, then it can legally be done. A doctor may provide such authorisation. Usually this is for the opposite reason, a baby younger or smaller than the minimum age will be allowed to sit forward facing, usually if severe reflux is an issue and aspiration a legitimate concern. There is no precedent (as far as I am aware) for medical authority to be used as a loophole to allow extended rear facing. But even though your son is very young, he is still older than the minimum legal age for forward facing, so a doctor may not find that there are medical grounds sufficient to warrant this unless he has a specific medical condition making forward facing unsafe.