OK OP. You don't seem to have received replies from the demographic you needed, and you'll not get one here either, but hopefully this will be more constructive for you.
2 things: Religious worship, and Religious Education.
a) Religious Worship
You can withdraw your child from worship, so any praying etc. This would include part of assemblies, graces before school, and anything else they do. It may be sitting in a side room, or just standing outside the hall for a short time. The school have to accommodate this, but your son may be the only one (you can ask this), and it may be boring, and he may feel separated. If you opt out of this you will also be opting out of nativity plays, carol concerts etc, which could be quite separating and distressing.
b) Religious Education
This over time should cover a variety of religions, but they are clearly starting with Christianity which makes sense. However, in RE lessons, they should be saying 'Christians believe' not 'This is true'. It shouldn't be necessary to withdraw from RE if they are giving it a balanced approach. Thi8s is worth raising with the school with a 'can you explain how this is couched in RE lessons, as DS seems to think it is being told as fact. Do you use the same wording for teaching Christianity as Islam'.
You have 3 choices
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Put up with it, and continue to give a balanced view at home. 'Yes lots of people at your school think XXX, we think YYY, other people think ZZZ, when you are older you can decide what you believe'. And repeat. But do check how they teach RE, it would not be unreasonable to ask for balance in RE lessons.
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Ask for your child to be withdrawn from Worship (and RE if you really insist). But you would have to take the consequences of your DS feeling quite separate from a lot of the life of the school, especially if he is the only one.
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Put your DS on as many waiting lists as possible and pray (!) that a space comes up he is eligible for. Younger siblings may then get in to new school on sibling rule.
hth