Maybe you could pass this on (taken from Kidsafe WA fact sheet:
"In Western Australia, the leading cause of death and the third most frequent cause of hospitalisation for children aged 0-14 years is transport related injury (2000-2008WA)
On average, 149 children (less than 16 years) are killed or seriously injured and about 221 will present to the Emergency Department of Princess Margaret Hospital as a result of injuries received as passengers in a car crash (2007 RRC & 2011PMHIS)."
Also there is a $500 fine and loss of 4 demerit points if he is caught.
And it is simply IDIOTIC and just unbelievable. I would go crazy if my DH did that with my DD. Hope you can get through to him. Maybe tell daycare and see if they can chat to him too.