Notsureabouthis - my DSS is similar, although a year younger than your DS. Not naturally interested in studying, when he does it he does it as fast as possible to get back to Fortnite, and so does a poor job at it. I check over his work and he's rushed it and got everything wrong.
PPs are saying end of Year 9 exams don't matter.... and that's true, in the long run the results don't, but I don't feel it's about the results. It's about young people learning how to learn - what are the techniques that work for them as individuals? What does self-discipline look and feel like? How do you practice it? How does your brain best retain information? It's not about the knowledge, it's about the process.
That's the one thing that DSS and I have found actually interests him - is using studying as a way of learning about himself.
We worked out that he doesn't focus in the afternoons - once lunch has hit his belly, his brain turns off. So mornings are study time - because his brain works better and in a more focused way, he gets his work done in less time and it sticks in his head better (which is a benefit for him to know, because then he gets more hours 'off' in the day).
We worked out that reading doesn't stick in his head - but explaining something to someone else cements it. Also he can't focus when he's sitting still - he has to move. So he walks laps around his room / connects ideas to locations around the house (Memory Palace-style, being in the hallway reminds him of the moment photosynthesis 'clicked' in his head, for example, and the living room curtains remind him of what he learned about Kandinsky) , and then comes and explains what he's learned to me in his own words.
We learned that short bursts with mini-rewards keeps him motivated, so he uses the Pomodoro technique now (25 minutes working, 5 minutes getting a juice or a snack, texting a friend, playing with the dog).
Don't get me wrong, he still would rather be on f$%^ing Fortnite every second of every day, and not everything is smooth sailing.
But by focusing on learning how to learn (and therefore get the adults off his back faster) we seem to be making progress faster.
I wonder if that might help you, too?