My DS was exactly the same, but with GCSEs. Intelligent, but pissed around and got awful results. He left school at 16, worked rubbish part time jobs, waiter, filling station attendant, retail - all until he got sacked for being late/not turning in.
At 22 years old he grew up. He went to college and did A levels, went to Uni and did a degree, did an additional year to qualify as a teacher and now, although he does not work in the teaching profession, he has a secure, well paid job with a large company, is married with a child, mortgage and is a happy, happy man.
I always felt guilty through the dark years, thinking we should have done more, pushed him to study, stopped supporting him and made him man up, but if you speak to him now, he will tell you that no matter what carrot or stick was waved at him at the time, none of it would have made any difference - he did it when it was right for him, when he was capable of studying hard and determined to succeed.
In contrast, other child sailed through education, university etc and has an equally wonderful life - they just got there in different ways.
I dont know what i am advising really, just wanted to let you know that there are eventual success stories out there.