I previously study in Adelaide. My high school, Glenunga High School (now changed to Glenunga International High School) used to have many HK students.
Public school in Australia is not bad. However, not all public school provide accomodation for students, some students will rent a flat or home-stay with an Australian family. I think that is good enough to let your son to be independent and learn more English.
The chance to get into University is same for Public school & for Private school, as long as your son study hard, he can still enter the University he wants to.