My DS went from a 3 to a 5. My DS has SEN and an EHCP. He has always struggled with maths and even now he still doesn't know his times tables - we've given up at this stage! Interestingly, like your DS, mine is much better at science which no one at this school can understand. I guess it's just the way some brains work?
Past papers are your friend. Be prepared to spend a fortune on printer ink!
You may already know this but if not, DC sit 3 maths papers: 2 calculator and 1 non-calculator. You can download and print off past papers from many years back also the mark schemes. They are free. You can get them on either the Edexcel site or somewhere like 'Save My Exams'.
What my DS's tutor did was get my DS to do a past paper as homework then they'd go through it in the session. I can only speak for Foundation but he will learn that the same type of questions come up every year eg - 'Bob needs to buy wallpaper/carpet for a room etc'. Repetition is the key.
I also second Hannah Kettle. She uses the same method ie doing a past paper then watching the video to go through the answers.
As an aside, please do make sure your DS has plan A, B and C for post 16. As others have said, you can apply and hold offers for several settings. Does your DS know what subjects he'd like to study post-16? Has he started going to Open Days? Where I am, a student needs at least a 6 in the subject they want to take. They may also need say a 6 in maths/English. All this will be available on the setting's website though.
It's very encouraging that your DS is willing and prepared to put the work in - that can make all the difference. Good luck to him!