FFS, what awful, judgemental and plain wrong responses the OP is getting here!
She's not asking for opinions on whether football is better than rugby, or advice on how to manage her family/time, how it will look on his CV or whether he needs to see a GP for low energy! And she doesn't deserve to be grilled on whether it's her son's choice or her choice! Give her some credit please!
She is simply asking whether school can force him to take part in an extra curricular activity.
OP, don't listen to any of the nonsense about conditions/expectations of the school. If it was a private school there might be a contractual obligation, but it isn't, it's a state school (grammar is irrelevant). A state school cannot force any child to take part in extra curricular activities, and certainly cannot force them to miss after school or weekend time doing so.
As one or two (enlightened) others have suggested, just email the school, tell them he won't be taking part. End of. He cannot be punished, he cannot be kicked out of school, doing so would be unlawful. I wouldn't even waste your time requesting/reading policies, there will be nothing in a policy that can enforce his participation.