The only faint hint of any idea from ours (a couple of weeks ago) was "maybe an academic". You have to read his school reports for the last twelve years to fully appreciate the irony of that one.
Being an academic rocks, Mike - it is the best fun I have had in all my many incarnations. Tell him to go for it - you get to hang around with trendy types like me discussing things we never have a hope of influencing and then writing about them for an audience of three in journals no one ever buys. People you meet say things like 'wow, you must be really clever' before glazing over totally and asking if you saw X Factor
. Which of course you didn't because it is too far beneath you.
PurpleDuck - it has taken me a whole lifetime to get to what I want to do. My 19 year old hasn't really decided yet, and she has also proved that if you get decent grades at A level, it doesn't really matter what subjects you chose as there is always a way into the courses you fancy if you are determined enough. Let him explore and he will think of something in the end - though if he is like me you may have a bit of a wait.