You are assuming that your son is missing out on a lot, but is he?
Is there any evidence that either the Christian or the Buddist classes that you mention are offering anything positive? He may well be better off in the library.
from the preamble to the petition.
While the standard curriculum certainly aims to educate children in values and principles, it is also acknowledged that those children attending scripture are able to have this work extended in lessons with a particular focus on this domain of life. It is unfair and unreasonable that some children be denied this opportunity for extension simply as a consequence of their parents’ choice not to have them attend scripture classes.
Let us examine this, there seems to be a presupposition that children attending scripture classes are gaining something and that those not attending are somehow at a disadvantage, but if they are at a disadvantage then it is the parents choice. If parents choose to knowingly disadvantage their children then their wishes ought to be respected, albeit condemned. Attendance at Scripture classes is not compulsory even if one's particular religion is represented. According to St James (ethics not apostle) only some 25% of students do not attend Scripture classes, this may represent more or less than 25% but it is a minority.
Further to the idea that children who have religious instruction are advantaged what should we do to counter-balance the further advantage that is being gained by children who attend Sunday schools? Perhaps we could have an ethics based class each Sunday morn in State schools for those who are missing out.