We were in a similar position last year with my son.
We decided to enroll my son to do an IGCSE Higher Maths distance learning course with the National Extension College (NEC). See:-
He started this during year 6 in junior school - he's now in year 7 in a Birmingham KE Grammar.
It's a tough course but the course and notes are really excellent and they also get a tutor allocated that they can discuss things with via email - it's also helped him a lot with maths at his Grammar school - so much so that he finds this a bit boring in comparison, but he does get good exam marks (A* in his year 7 end of year exam)
Might be worthwhile considering, costs about £290.
PS my daughter who is 3 years older and taking IGCSE Higher Maths next year has been using the material to revise - she says the material is very useful!