As you are in Australia and many of the folks on this forum are in the USA it might be hard to give you decent advice. That's a nice Alan, with good components. Those were neat bikes, and are good riders. It will likely need going over, possibly new cables, likely new tires, etc. It should definitely at least be cleaned before you try to sell it. Here in my area of the US that might bring USD $500 to $600 cleaned up and ride-ready. As it is it will appeal primarily to the bottom feeders who will want a bargain -- you'll probably see some of them here saying USD $200- $250!
No idea what the market is in Australia though. If your son is actually interested in riding it that's the way to go. Good luck.