The good news is he is in year 10 and not currently sitting them.
Advice on the internet is free so find something that makes sense to him. Youtube is great for English. Try either Mr Bruff or my personal favourite Mr Salles, make sure of the exam boards the school use, probably AQA but double check.
The narrative he will be expected to write, he can look ahead and have things prepared, not a story as that would be impossible but descriptive words for weather or places like a school playground, the woods, a back garden. Google good ways to describe a sunny day, in primary we call it magpie, you take the words you want. You could help with that.
History, if he is AQA then Gerard O'Toole on Youtube is great, lots of silly hats to help with memory, mine did a different exam board and I just watched him for fun because I love History.
The school also provided mine with WAGOLLs (what a good one looks like) that highlighted the AOs in different colours (assessment objectives, both English and History have these) firstly they will be in his school book, also the revision guides and helpfully on past paper mark schemes.
All these past papers are online, for all subjects. As an adult it is much easier for you to figure out what he needs to do. It literally lists out what answers they are looking for under each AO. Help walk him through an English paper, both lang and lit and look at the first question, then the answer. Next question, next answer. Don't do the whole thing first.
This isn't about hours of 1:1, this is teaching him the tools to know what gets you marks. Sadly, this is important. You don't need to know everything yourself. Depending on what fiction he is studying for English, find the film/play on youtube. The revision guides walk them through it.
He also has a whole 6 weeks of this summer to work on things. He will be awake for over 12 hours, committing to doing some work broken up over the day will help him tremendously. Reward him for his efforts, not his grades but if he gets back on track with maths he will never have to sit the exam again, that is usually a good motivator.