OP, I teach and can tell you with absolute sincerity that there is a correlation between those children who are allowed to skip school to avoid something they don't want to do in Y7,8,9 and those who struggle cope with any unwanted expectation by Y12.
I think it is sending him a very negative message indeed if you keep him off, and setting a precedent.
If everyone is taking part then there will be many, many children dreading the day, taking on minor roles and doing a bad job of it; he certainly won't be the only one feeling like this.
By sending him in you give him the message that you support the school and their decisions about a rounded education, that it is good to overcome your fears, that non-attendance will let down his group.
As pp have said, some children feel like this every single day about maths or PE or whatever, but have no choice in the matter.
Even if he hates it, he will come home relieved and knowing that he can get through anything.