I just registered specifically to post in this topic. I'm not even sure how I got here to be honest, but I was bored and was quite interested.
Your son sounds like me. I am 24 now, but at A-Levels I was disinterested, he sounds like a typical underachiever - like me. Even before that, I got 2 A's in GCSE and the rest Bs... Just coasting.
At A-Levels, I got BBC (compared to my younger brother who later got AAB and A*/A in GCSE), wasn't disappointed, didn't really care to be honest. I'd applied for a course in University just to make my parents happy - but had a gap year doing pretty much nothing.
All gap year my parents were on my back, cut me off financially, but still paid for food and gave me board. I eventually got a job in a bar and worked enough money to see mates and so forth. Come the new year, I was reluctant to go ahead to university - Keele University (the underachiever's uni...) to read Law and History (my dad is a solicitor and I enjoyed history at A-Level - pretty much the only thing I enjoyed in High School).
Long story short, I got my LL.B (2.1), went on to do a Masters in Law (LLM) with Merit, got my dissertation published, and am now working for a popular budget supermarket chain in their head office, on a starting salary of more than £30,000.
How did this happen you might ask? Well, essentially, I wanted to keep my free room and board. So I got a job. I also gave into their cajoling to apply for university. Once there, I thrived in the 'halfway house' of university - the period where you aren't totally moved out, but aren't living at home. My academic and social life picked up and I am now a successful professional.
Bottom line is, parents, be persistent. Don't worry if your son/daughter storms out the room looking angry - keep pushing them. Maybe, once a week bring it up, but push it until you see them getting a little annoyed - but don't force them away. It is a fine line between persistence and aggression.
As I said, I don't know if this will be relevant, but it seems this site needs posts from the other side of the fence, as it were, to put things into perspective.