@LizzyBizzyLizzy Talk to him about the future and how he is going to afford things like if he drops his mobile phone and it dies, will he be able to pay for it? Same for latest games, console, headphones, nights out with mates, clothes, shoes whatever is his currency, something he understands.
With mine I used teachers as an example of a job you need a degree for, they can relate to it. I sat with them as they looked up teacher salaries, ran that through a mortgage calculator for them to see what you can buy locally on that salary. Then hit them with all the outgoings they will have, some they will know such as gas and electricity but what about council tax, house insurance, broadband, how much food costs for supermarket shopping etc. Will they want a car? Insurance, servicing, petrol. Now is the time for them to know how much a car costs, a holiday. Tell him you want him to have all these things, to have a great life with money to spend.
Now do the same with minimum wage jobs and also look up Spareroom for rooms to rent in your area so they can see the rental market too as well as house buying. This is all leading to the better you do at GCSE the better your A level results will be and if you want to go to university then do you want all doors to be open to you or closed? If you are predicted BCC then that rules out a lot of university courses. Predicted AAA then you have far more choice.
Jobs wise, if he wants to work, who will they employ? Who will he be up against and will his grades and effort make him top of the pile? Explain the grade curve that grade boundaries move every year, the more people who do better than you the lower your grade. That means some children sitting a GCSE will be forced to get grade 3 or below. So when he starts GCSE content start making revision stuff from it, make flash cards as he goes along to save him a load of time at the end. Plus he will be tested on these things as he goes along so he will have already prepared revision stuff whilst it was fresh in his head.
We rewarded effort for attitude to learning at school, engagement in class and with homework. This went hand in hand with general attitude at home, mucking in when required, doing gardening work, tip runs, unpacking supermarket shopping etc with no complaints. They then get gaming computers because they have earned it. I know this might not work for some children but surely worth a try. Remind him that you love him and are willing to have him be pissed off at you for making him work because you can see the bigger picture.