Hi Kat and welcome to BE,
The most outstanding private boys school in Sydney would be Scots College, at Bellvue Hill. http://www.tsc.nsw.edu.au/
The surrounding prestigious suburbs of Double Bay and Woollahra are home to lots of Sydney celebrities, and of course have a very high price tag.
On the beautiful north shore there are both Newington College in Lindfield (on train line) and Sydney Grammar in St Ives. Travel into CBD would be within an hour.
Another very good one is The Kings School in North Parramatta. Lots of nice surrounding suburbs, and maybe the least expensive of those mentioned. Travel by train would take approx half an hour into CBD