OP, has your DS always been weak in Maths or have they suddenly declined?
If it's always been a weaker subject, then think VERY carefully about them doing it at A level as it's incredibly hard and there's a huge leap from GCSE to A level standard. A grade 7 is still not good for A level and some schools wouldn't let someone with a 7 onto their A level course which gives a good indication of where the pupil should be to be capable of succeeding at A level.
If it's something recent that's happened and they'd previously been working at top levels in Maths, then, yes, maybe 2 tutors may bring them back up to speed and then maybe A level wouldn't be so much of a stretch for them (assuming they didn't get the same teacher they don't understand in sixth form!!).
Similar happened with my DS. Maths had always been his best subject, always near top of the class at primary and early secondary years, but then one year mid secondary school, he "crashed and burned" in it with, what he says, was an utterly crap teacher that couldn't explain things. He went from something like 90% in the prior year end test to something like 40% the next year - very demoralised. We didn't get tutor, but me and DH "tutored him" ourselves during the Summer holidays - basically bought books, revision guides, practice papers, etc for that standard, and re-did the year ourselves. He started the next year much more confident, different teacher, and by the end of the year, back up to circa 90% in the year end test. He went on to grade 9 at GCSE, A* at A level and a First degree in Maths at Uni.
So, sometimes, extensive "tutoring" can help, but really only if it's to tackle a particular short term problem rather than a general weakness.