Using someone's wifi without permission is definitely stealing. I'm amazed that anyone thinks it isn't, or that it's somehow more acceptable than walking into a shop and just taking things because they aren't "locked down".
That being said, OP it's not impossible that your son has connected legally to a free public wifi in your area. Lots of places have them these days, plus there are the Telstra hotspots being built in the old phone boxes. Check on your phone and see what wifi networks are available, if there are no free ones in your area he could be stealing it from a neighbour.