I view any time outside of normal school hours as my family's and my time. We get to choose what we do with it and neither school nor anyone else have any right to dictate what we do with it, if something doesn't suit we have to say no.
Because of various problems with an A level DS1 was doing over the last two years he was 'requested' to go in for extra lessons one day a week for half a term, from beginning of school until 5.00pm not just the end of the school day. The letter was worded in such a way as to make it sound compulsory.
We did appreciate the fact the school were offering extra tuition. I signed the permission slip but put a note on to say that when we had other things on he wouldn't be able to stay. No arguments from school thankfully.
It was the first time this school had taught this subject at this level and it had a lot of teething problems. Every student had been estimated to get A grades or higher and if you don't look likely to hit the estimated grade you are 'offered' extra lessons. Things were missed in the teaching and it was a bit of a disaster all round so all the dozen or so students in this subject were supposed to attend the extras, it happened a couple of more times over the two years too. Not every student turned up for the extra tuition, but there wasn't much school could do when it involved time after school, people have other commitments. All they could do was revoke the students privilege of leaving site over free lessons and have them study then if they felt it necessary.
Ds1 did miss a couple of sessions because of other appointments and he forgot about the first one he was supposed to stay for at the beginning of a half term, so he missed some and school didn't argue about it.
Unless you commit to something then it isn't compulsory, even then if something happens to change your circumstances you can pull out of a commitment. Ds1 has aspergers and if he was offered an hospital appt when school was wanting him for the extra tuition he would have gone to the hospital, it would be harder to get another appt than catch up at school.
In our current circumstances dh only has 1 in 3 Saturdays off work, we make plans for those Saturdays and would not make a child go off with school to do something they don't like doing when we need that one Saturday every three weeks as our only family day together. Even without limited free time it would be a no from us for sending any of our boys off to do this if they don't want to.
Your ds doesn't want to do the activity, you are happy to not make him do it. Don't force him to go