The "old Timers Mine" in Crowders Gully Rd would be a good mine for your son.
They have a self guided tour underground in a 1930's vintage mine which should satisfy any little (or big) boy, and twice a day they have a free demonstration of the methods and gear used in the mines out front of the building.
Seeing the big "vacuum cleaner" devices to get the muck up actually working would be a good sight for him
old time mine coober pedy by boo 53, on Flickr
As long as it hasn't been raining recently you should be fine, and I'm sure they'll have signs out if there is a problem