Under the Australian system at that time, If he received a "school certificate" and left aged 15 it was at the end of year 10. To go to university you need to finish year 12 and get a "higher school certificate" in order to matriculate. It seems odd that he would have been offered a university scholarship with only a school certificate.
I loaded up year 11 with extra subjects and received my higher school certificate without going to year 12, but was the youngest person when I entered university. I still needed to get the higher school certificate though.
Who knows, maybe they were scouting for talented youth, and offered him a potential scholarship, but he would still have been required to finish his HSC before getting into university.