Most sixth forms that offer a levels want to make sure that the students are on the right course for them.
That means that they want the students to be both academically capable of doing the course, and for want of a better word, personally capable - ie will do homework on time, doesn't disrupt classes, will do the level of independent study needed.
So both state and private sixth forms have grade requirements to ensure students are academically capable.
Many will also do an assessment maybe a month into the course to see if students are basically managing to keep up.
In my school this is done quite early and then the students whose results show that they aren't keeping up have conversations with the head of sixth about potentially changing a levels (eg dropping further maths, maybe dropping down to BTEC etc)
They feel that a student who isn't keeping up after the first month is very likely to walk away from a levels with bad results, and they'd rather that student was on a course where they can succeed.
For whatever reason your son's school think he won't keep up. So maybe worth thinking about what will be your plan B if he's not keeping up.