There seems to be so little hope with that lead, interested.
With it taking six months before anyone reported the sighting to the police, I can't imagine that even highway CCTV records for 12 Sept 2014 would have still been available, but maybe they were.
One would hope that from March 2015 until now, the police might have been able to narrow down the registered beige boxy Land Cruisers in NSW at that time to a suspect vehicle or two. Same with the iridescent blue Ford. But if the vehicle(s) has gone interstate or been dumped into a lake or didn't come from NSW in the first place, what then?
Would William even remember who he is, if he is still alive somewhere? More than ½ of his life has passed since he was abducted. And what kind of life may he be leading?
That lead seems to be the saddest missed opportunity, if the child was in fact William.