Try a different approach - agree that he doesn't know what he wants to do. So that means he has to give everything a try, a good try, and that will help him work out where he wants to go.
So - is he hands on, a reader, or a watcher ? Maybe say trawl through the documentaries on iplayer - loads of series on BBC4. Say every week watch 2 programs on history (can include things like who do you think you are), two on science (eg horizon, science club), two on nature/geography, and 2 on current affairs. He can chop them into half hour chunks if he finds he is drifting in a hour long session.
Languages ? - lots online - pitched at all levels - a bit of googling is required.
That will broaden his horizons, give him some good general knowledge and hopefully point him towards what he wants to do.
And a pep talk. He is old enough to see adult hood on the horizon. He has to realise no-one is going to feed and house him for the rest of his life. He is going to the one responsible for himself. It would be nicest for him if he is doing something he enjoys rather than a very boring low paid job. His job now is to give himself the best foundation he can - which is broad.
Arts, crafts, sports, music - can all slot around that to make life more enjoyable - and if he is good at one - then maybe a future route to earning a living. My lot also love tinkering with wood, metal and stuff to make things.