It has been a strange year and, with exams cancelled, the grades are only representative of what his school covered. Hundreds of kids are in the same situation so please reassure him about that. He will realise that in A levels the classmates are going to be those who loved the subject AND who are good at it so he may find himself at the top of the class in a couple of months or realise he is just average among other maths lovers, but that is not a bad thing, he is still as good as it was before joining the new school, he just needs to catch up a bit. I strongly recommend you get the study guides for each subject so he can start catching up on his own.
My son also moved schools for A levels, and experienced the same at the beginning. I am grateful that the headteacher mentioned at the open day (mostly attended by their own pupils and a handful of prospective newbies) that most kids find they are “not as good” in A levels as they were in GCSE, and that is not because they are not as good but because they have not realised that to get the same grades they got in GCSEs, they do require to study twice as much time. DS school required 2-3 hours of self study for each hour spent in the classroom.
I work with university students helping freshers to set up, believe me, most students say that the big jump in hours of study needed was not from A levels to Year 1 but from GCSE to A levels.
There is a lot of work to do, but please reassure him he is not feeling lost just he changed schools or his previous school didn’t taught him that well but because the work is far more demanding than A levels.
But again, to reassure him, get the complete study guides from Amazon, just ensure you order the right ones for the exam board curricula followed by the school.