OP, this is the million dollar question. I think the best person to answer it is probably DS. What would help him?
My own approach is to let DD be in charge of it herself. I am available to help and have only just been required in Year 10 maths but I do occasionally ask what level of homework she has and when she might do it. She is pretty studious, as was DSD1. DSS was more like your boy, I suspect, although there were no tutors involved.
After DH trying to provide assistance it was agreed that he would desist and wait to be asked. This made everyone's life better. His results were surprisingly alright. He didn't revise at home for GCSEs but must have done enough at school to get through, and did much better than expected, if we're honest. We were braced for retakes. He worked full time for a year before managing to get an apprenticeship and is now qualified.In our case, all that would have happened if we'd done anything differently is that he, and we, would have been miserable and in conflict. Home should be a nice place to be, I think.
I do a job where I have to do some work from home after hours. I'm fairly motivated but I don't always feel like doing it, sometimes I leave it to the last minute. However, if anyone I lived with asked me when I was planning to do it or suggest I should do it instead of having a lie in/doing sport/seeing friends, it would be intolerable.
Talk to DS and agree what support he would like from you, and do that. What does he fancy doing next year? What GCSEs does he need for that? We were very clear with all our kids that their education was theirs. DH and I have got GCSEs already. It wasn't about us. We took them out to celebrate at the end of the exams not when they got the results. We praised effort and other things. They are in school all day with teachers suggesting these results are life changing. I think coming home to the same pressure would be really difficult, especially if you are not especially academic.
It is really important to keep friendships, sport and fun going during exam years. Sitting in your bedroom staring at the same piece of paper to appease a stressed or potentially disappointed parent is a poor use of youth. I think you learn more in small chunks and he is already doing tutoring. Trust him and help him if he asks. Make sure he eats well, gets some sleep, has some fun and sees himself as good enough and you've done a cracking job as GCSE support crew.