Been there and am still wearing the T shirt!!
My DS1 has put us through the mill on and off since he was about 12... vile wall smashing tempers aged 12-14, then smoking, then the complete lack of revision for GCSEs... lying, stealing (from us).. yadda yadda.
It's hard ..really hard. I spent an awful lot of time wondering what the heck I did wrong, and why DS1 had gone off the rails when his two sisters and brother haven't. I attended a parenting course for difficult teens (which actually cheered me up..OMG some were so much worse)
On the exams front, I made myself seriously unpopular with DS1.. I came down on him hard. When his teachers started ringing me to tell me they were concerned he wouldn't pass his GCSEs as he did sod all work I clamped down, printed off every past paper from the internet possible (each exam board has these) and he wasn't allowed out until he had done 2 past papers a day. He HATED me... right up til results day when he passed everything with C or above.....
It hasn't got a lot easier.. he's doing a Media B tec, but managed to get himself curfewed by the local police for misuse of his moped (bought to get him to his evening job!). The curfew has been GREAT and I wish it could continue , but he still is basically a bit of a prat. He IS working 5 evenings a week at Domino's Pizza which keeps him off the streets but we recently discovered he is smoking pot... It feels never ending.. but I hold on to the hope that with patience and firm boundaries he WILL eventually grow up...in the meantime we limit the damage (and in the case of smoking pot..I've confiscated his bank card so he can't spend anything....!)
Some boys really know how to test the sanity of their parents. My DS1 is actually a loving and gentle boy, who is fab with his autistic brother, cares about his sisters, and can be just lovely...and I hang on to that!
I second the siblings comment.. my DS1's two sisters are high flying academic, well behaved easy going girls who have given me the minimum of stress... kids are just different.
Try and keep the lines if communication open..talk when he WILL talk and don't be afraid to impose boundaries. My son has been a pita despite them but I have no doubt they have helped!!