I spent years trying to get my son to study, and forced him to stay on to do Highers. He knew exactly what he wanted to do in life and worked for free helping local companies/community centre after phoning them up himself and talking to them.
He used to say "None of these exams will help me in this industry, it's all about your experience." He was absolutely right. He left school at 16, got in touch with companies far away (where the industry mainly is) and went off to work. He's been self-supporting since 16, has his own little flat and at the age of 20 set up his own business. I think the point is that you can leave school early/with not great qualifications IF you're really motivated/have the drive to succeed in something and the social skills to make the connections. That's rare in a teenager.
Find out what your son imagines life will be like after school. If it involves a fancy car, holidays, active social life you could cost them out with him (a nice maths lesson) and look at what job he will get that will fund them.
An apprenticeship may be more suitable for him if he can see where it would lead to, or a college course that could lead on to Uni if he chooses to (however College courses can have surprisingly high entry requirements - it would be worth showing him) . It is about the end goal, not the exam results in isolation, but good results give him so many more choices.
So many of the teenagers I worked with had a vision that when they left school they were going to suddenly have a car/money/cool job. They had no real idea of how they were going to achieve this or the real price of things. It is sad that for so many, being able to see the connection between good results and good career comes far too late for them.
I feel your pain, I found it incredibly stressful/frustrating because I felt like it was my responsibility to somehow make him study/care. Meantime he was chilled out/couldn't have cared less. All the stressing made zero difference to his motivation to study.