OP are you able to tell us what his UMS was and what the boundary mark was?
As others have said, he may have scraped the A grade and this could be relevant.
In our local school, to do A Level, students need to achieve at least an A in the subject they wish to study AND have to be green lighted by their GCSE teacher about their ability (can they achieve a B or above at A level) and attitude to work. It is possible for someone to get an A and not be green lighted if in the teachers judgement the pupil doesn't have a good work ethic or has limited ability - it recognises the vagaries of the exam system and puts teacher judgement ahead of grades.
Student A Level choices are analysed closely in Yr 11. If they are picking things which might result in lower grades than if they picked other options (only academic subjects offered) then this is pointed out to them.
Is this because the school is looking out for the kids best interests or the school league table position? A bit of both. These kids want to go to RG Unis and in most cases will need a B or above in each subject to get their offers and places. They are guided to was subjects where this will be possible for them and away from subjects where a C or below is the likely outcome. Yes, this will boost league table positions, but it will also allow students to get to the good Unis.
As far as I have heard, anyone who isn't likely to get a B or above in anything is offered a different course to the 3 a level standard. I hear that some do just 2 subjects or do a BTEC with an A level. Some choose to leave, but I dont think anyone is forced to leave altogether. And I don't think people have to leave after Yr12 if they do poorly, although I'm not sure, but I think that once on the course they support the kids through to the end and provide extra help for those who need it.
OP can you tell us what marks your DS got and also tell us if he was perceived as an able and hard working student at GCSE or if there were issues lower down the school which might account for refusal to take him. And I'm glad he's found somewhere which will take him if he's really set on A Level maths. Would you rather he did it and got a C grade which might limit uni options or prefer he did something else where his likely grade would be higher?