He has always worked at level 5/6.
Ah, this sounds like confusion about reporting of levels. He can't have always been working at a 5/6, what they've been reporting is that they think that's the sort of grade he'll be getting at GCSE. It is, however, very difficult to predict the future, especially during the last 18 months when decent data on students has been thin on the ground.
It sounds like he has sat some proper end of year exams at the higher level and they have got a better idea of where he is at and think that he will struggle at higher. Do you know what his results are and how he found the papers?
If he struggled quite a lot with the higher papers, then it is a good idea to make the switch to foundation now, so that he is properly prepared to sit them. If, for example, he aces his mocks on foundation (presumably around Nov Y11) then there is still time to argue that he should be moved back to higher before the final exams.
The foundation and higher papers overlap by 25% (the grade 4/5 content) so he'll be working on content that will be on both papers.
and if he is expecting to get a 4 or 5 what’s the lose in doing a higher?
I've got a class who have just sat higher for their end of Y10 exams. Some of them will be moving to foundation as it's clear that the paper isn't suitable for them. It isn't good for students mathematically or psychologically to be sitting a paper where they're getting a very low percentage.
There is always a debate about whether it's easier to get a 4/5 on higher or foundation, I would agree with your DS's school and say foundation. It's certainly a better experience for them to sit foundation.