Hi Byron, and good to hear you've settled into Sydney, some great areas there and St Ives out the northern side a bit from memory shouldn't be too bad.
A 15 YO has no doubt left his mates behind and so I suppose you need to reinforce with him what his basic interests are.
Any sports? - cricket, soccer, rugby, even Aussie rules, surfing and then a great atmosphere can be had with the Surf Life Saving Association and he may want to join a club, some even having bunkhouse facilities so he might be able to have the odd night away in a controlled environment.
And what about camping, some great places real handy to Sydney, The Basin for instance in Kuringai NP right on northern suburbs doorstep.
Then there's sailing and many yacht and sailing clubs about the harbour where he should be able to join up and learn to sail.