I can recommend FreeScienceLessons for basic science concepts explained well in short 5 minute videos.
Primrose Kitten and MyGCSEScience also good, but longer and go in more detail.
Make sure he goes through his Year 10 exam paper and identify where he lost his marks (teacher will usually do this with him) then print off the specification for Paper 1 and colour green/amber/red for each topic.
If you print off the multiple choice questions for Paper 1 on past papers going back a few years then mark them from the mark scheme (you can find both on Maths Made Easy past papers if you google) you can see which he struggles on/gets wrong and that also gives you the topics/areas he is weak in to work on.
See if you can persuade him to make some notes/flash cards on these in the holidays as there's still a lot of the syllabus to go in Year 11 and if he ideally gets on top of Year 10 work he will then have time to work for Year 11.
Biology has the least maths/calculations so there's no easy marks for a science if he is good at maths. Sciences are content heavy so it's worth spending a bit of time consistently-say half an hour- a day on it.
I would say if he's getting the grades in his other subjects, then it's probably lack of knowledge that dropped his grade and he needs to spend some time concentrating on purely science to get it up.
The only way he will be sitting higher paper is if he gets 6/7 in end of unit tests/mocks from now on I would say, as there's still a load more content to come.
My ds was stronger on Paper 1 on all his sciences because he was able to spend longer revising in Year 10 than Year 11 where his other subjects started to demand more of his attention.
It's hard to work on your weaker subjects so a tutor should bring some enthusiasm into the equation, but it needs work on your son's part learning the content to do well. The tutor can help the parts he doesn't understand and exam technique.
Sorry for the essay, but hopefully there's something here that helps.