I thought I'd offer a slightly different perspective. I didn't do further maths but I do have a maths degree from a Russell Group university. We caught up the further maths curriculum quite easily in the first term of the degree. Maths is a bit like that, if you are lucky enough to be gifted at it, it is very easy, if not, it is a slog!
Naturally gifted mathematicians (someone who has always been at the top of class/ year group and who can get the top grade - a 9 - without hard work) won't struggle with further maths but personally I preferred not to narrow down my options at 16.
Good luck with the grades - hopefully it's all been sorted now and your DS will get his 9.