Is your DS in a small school that only has a limited range of options?
The schools in the city where we now live [not the city in my name] generally have between 1300 and 2000 pupils and 250-300 per year and they generally all offer a wide range of choices and only the church school actually insists on pupils taking a RE GCSE exam - all the other schools here just have one period a week for those not doing it to GCSE in order to comply with the law.
Just as an example of what they can choose:-
They have to do Eng lang, Eng lit, maths, either core and addit science or BTEC science.
Then they have to do one of history or geography and choose three further subjects which can also include history or geography if not already chosen.
They can only choose to do triple science or two languages if they are ''invited'' to do so.
So, certainly locally to me there is generally no compulsion in the schools to take RE as an exam and they do - at least in this school - get quite a wide choice of subjects.
Perhaps it's the size of the school that is also an issue?