Legal or not, it sounds like you are on a hiding to nowhere. Get out now. It’s not like there’s not cheap rentals in WA at the moment!
Fact is, you have signed something, and it’s not going to be worth your while either monetarily or in terms of your time to challenge it.
Only other thing I can think of is to be overly friendly, and either:
a) usurp the son in the affections of his parents, and maybe get in their will;
b) get their mobile numbers and constantly send little “hellos” or “I made a cake – want to drop around?” or “there’s a lightglobe out and I can’t reach it”. Hopefully they will think you too weird to be around; or
b) make a move on your flatmate’s mum. Might dissuade the parents from visiting…..