Been thinking a bit more.
Rather than you lobbying for your son being kept down a year, I feel that what you need to be asking for is for mechanisms to ensure that he makes as much progress as he possibly can in the second half of Year 6.
The thing is, SOME schools - by no means all - react to the pressure of SATs and league tables by focusing a lot of effort on those children who are borderline )just below the 'magic' level 4b and Level 5 benchmarks. For Literacy, your son may benefit from this as with his rate of progress in Year 5 in particular they may think he is capable of reaching Level 4 and so puch him really hard.
However, for maths this may work against your son as they may have decided that he's 'too low' and so effort may not be put into him making maximum progress as he may not achieve one of the 'magic' benchmarks.
I think that the question that you should be asking the head is how they are intending to maximise his progress for the next term and a half, particularly in Maths, as he has shown through literacy that he is capable of making extremely good progress even within a single year. I would be expecting to hear information about 1 to 1 work, specific interventions etc and a target of 3b for the end opf the year with a very clear plan how to get there.
And I wonder whether in this case - and i think it's the first time I've ever advocated it - whether getting some 1 to 1 help outside school with a Maths tutor might give him more confidence? if not a 'live' tutor, have you checked out the various Maths sites online that can help him practise basic skills?