There is a Facebook group called Find A Tutor (UK), which now has nearly 250 members, many of which are independent tutors i.e. not affiliated to an agency (and hence you know your whole tuition fee will be paid to the tutor). Send them a join request and post on there. There are a lot of maths tutors from across the country who are members.
What was his end of year result? Also his predicted grade. If he is working towards his target then I don't think he needs lots of tuition, maybe once or twice a month, unless he is behind. My husband and I run a tutoring agency called Impact Tutors. You can enter your postcode and the subject in search bar and then find a list of tutors available near you. You can find out about each tutor's fee, qualifications, teaching experience on their profile on our website and contact them directly. I hope