@Realisticparent I would say that this comes down to a few things. There is learning the content of a subject and understanding it, there is memory recall and then there is exam technique. Look back over his year 10 exam papers and look at where he didn't get full marks on a question and work out why.
It also comes down to attitude, whether they rise up to the challenge or not. Ds1 decided predicted grades would not define him, he revised effectively because he worked out what worked for him and I tested him on his flashcards etc. But the school were also amazing in their teaching. The way they graded was if you were a minus grade you would scrape into the bottom of it, just the number in the middle and a plus you were near the top like when we were at school. So end of year 10 he was "predicted" 7+ 77 7- 6+6+6+ 66. What he actually got was 999998887.
I will admit that I saw my job as his parent to tutor him through some subjects so English lit and lang and tested him for History a lot as there was a lot of dates/events. Dh sat with him for science. That meant understanding what the exam board is looking for, so I looked at past papers for English lit and lang, looked at each question and the answers to see what scored marks. I helped him prepare descriptive sentences to use in whatever narrative he would write. You can literally google good descripive words for weather and just nick that. They are 16 they are not expecting the next best seller from them. Understand the system. For poetry if they are doing Power and Conflict, Mr Salles 5 key quotes on Youtube and why to use those massively helped have things prepared. Know about form and structure and find out what that is, it is probably already in his English classbook.
For History he had all the past paper questions from school and then made an simple essay plan for each question so he knew which way he would argue something, so agree statements and disagree statements plus conclusion but making sure they hit all the AO1s, AO2s (assessment objectives, these are listed for English too in the mark scheme) but he prepared a lot of this over the summer of year 10 when he wasn't also learning new content. Your son has time, just commiting to doing a bit every day. Go back over the year 10 stuff. Good luck.
And before anyone says I am over invested I wish I had parents who were invested in my schooling. Ds excelled at A levels because he wasn't doing subjects he was forced to like English anymore and didn't need any help from us and is now at uni.