I'm in Vic (and my eldest is only 4), but from what I've read so far, if your son is already in school you should probably go to a GP and get some sort of medical exemption from school to cover the time between taking him out and becoming registered for homeschooling. If he's that far behind in his schooling I'm assuming that a GP would be able to give an exemption like that for some sort of mental health reason... That should cover you legally if the Department of Education decide to chase up his school absences.

Best of luck on your journey. I hope things improve dramatically for your son and that you both find homeschooling really rewarding