Your son gained a place on this course and is owed a duty of care by the people who selected him.
Umm, no - that's not how it works. They don't have a duty to stop anyone who might possibly fail from doing the course. If they did, they would have no students.
If he's ungainly now he must have been ungainly then, when he applied. The course has no excuses, therefore, to say he's no longer suitable.
They might have thought he would improve after taking the classes.
Being thrown out for being gangling is no different to being thrown out for being disabled.
No, it's really not the same thing.
I'd attack this course on the grounds that it's being discriminatory in it's marking process - your son can't help being tall.
That won't work - A) he's not being treated unfairly on the basis of his height, he's just not good at the movement aspects of the course and B) in any event, height is not a protected characteristic under the Equality Act (it is at most indirectly linked to sex as men tend to be taller, but I can't see a sex discrimination claim working here). Honestly, I can't see a remote chance of a discrimination claim succeeeding unless the OP's son has a disability which she has omitted from her post.
ask them how they'd mark a wheelchair user or someone with cerebral palsy. If the say they'd be able to compensate for that then they're being discriminatory.
No. Don't do this. That's not how the law works. Sometimes reasonable adjustments can be made for disabled people - if this is done, able-bodied people can't complain that it is unfair that they are not getting similar treatment.
Sorry if this sounds harsh but I just don't think you'd get anywhere with this.