I don't think the OP was at any point insinuating that a child attending a comprehensive school is automatically lazy, unmotivated or in any way inferior.
surely she merely talked about how frustrated she was with her ds lack of effort and motivation, and saying he is not earning or at the moment deserving of a place in that school, and somebody else, who was more hard working would therefore be more deserving?
Don't understand what's offensive about that.
Anyway, op, I understand your concerns. I think if he is otherwise doing 9 or so 'acceptable' subjects, i'd let him do it. He needs to learn to m ake decisions and to live by them.
I was at a girls' grammar school, and our head teacher, a strident feminist really discouraged us from doing anything 'soft', Home Ec was truly scorned, we all had to be scientists and engineers.
I would have loved to have done something creative, envied the boys in the boys grammar who could do things like wood work and metal work.
I've always regretted my lack of creative education. Think that's where my okra urges spring from! If you really want him to do RE, is the another area you can persuade him to change?
I suggest you or dh take him out eg to a restaurant or bowling, just 1-2-1 for a treat and an adult chat in a friendly and neutral environment, show him you're listening to him, and go through the list, accept his choices, but make it clear they are his choices!