Some of the posts contain sweeping statements about what is mandatory, what schools provide, what parents provide etc - I can only give my account of my family and the two schools we have been involved with so -
My DS primary was a very mixed and tolerant school until a new head took over. Up to that point the teachers covered many different religions and the associated ceremonies/festivals - something I and the families (at least the ones that expressed an opinion directly to me) loved about the school.
It indeed encouraged inclusivity and tolerance
The new head brought with him a huge focus on religion and not all religious, not even just Christianity, but a very narrow part of Christianity (he is from a very narrow, what I would term batshit sect) this included telling teachers to let unqualified church members in to take RE which became very much more instruction rather than studies or education including telling the children that those unbaptised would 'go to hell'
As my son was nearing leaving we expressed our disappointment as did many other families - going to LEA and even to ministerial level - but he stayed on for the last year while others moved school as I had sufficient time and resources to offer additional teaching at home.
Move to grammar. I asked to see their proposed curriculum for RE and despite being told he couldn't opt out unless Muslim or A.N.Other religion (misinformation easily debunked) I did pull him out of RE and substituted my own curriculum that covered many more religions and world views than their proposed one - lots of work but I believed it important enough to me and to his education to do so. (Btw he would have been stuck at back of classroom otherwise although a child of another religion was allowed to go to the library tsk!)
NI has particular issues concerning religion so I wouldn't have him devoid of learning and yes, perhaps I was better placed than many to give him such an in-depth and wide-ranging alternative curriculum (including exams), but I do not think it can be said of all schools that the RE lessons are unbiased. His deputy HT at grammar is also from a narrow 'sect' and used to be head of RE.
As stated upthread, asking questions about what will be taught, how and by whom wrt religion is a fair enough suggestion especially if you have the inkling that he may be being taught in a narrower way, more heavily weighted to one religion or strand. Then you can make up your mind as to pull the child out of collective worship or RE classes or both
I would suggest it is very much in your child's best interest to have some form of substitute teaching in RE though or you may leave them unprepared when faced with religion at a later date (fore warned is fore armed and all that)