I agree with posters pointing out that hours of ‘revision’ doesn’t means hours of useful work- particularly if that is reluctant work. I don’t want to hear my students are doing 3 hours a night at this stage as it is highly unlikely to be productive and much more likely to be contributing to unhelpful anxiety.
It is great that you are thinking of his GCSE results but i would suggest that there are more time efficient less confrontational ways to show support and get him working.
Have a proper sit down with him and get him to rank his subjects in terms of which ones will need the most revision .
Agree a plan together and then suggest a reasonable timeframe.
Point out that the work he does now will be work the Easter holiday him will be very grateful for as he will be so much less stressed. Get him to discuss what grades he wants to see when he opens the envelope in August and what he wants to do in 6th form. Try and take the head butting element out of conversations .
Lets say you agree an hour with 20mins or so on 3 subjects . Help him decide the tasks he will do Get hold of past paper practice books and get him to do a couple of pages properly, thoughtfully and mark them. Do a section of Seneca or similar. Tick off what he has done to see progress .
Most schools are doing mocks now as a benchmark to see what work needs doing. See how he does, go to parents evening and ask his teachers.
Your support can make a huge difference to his grades but it will be a long 6 months if you are trying to drag him with you rather than find ways to motivate him to step up himself.