I haven't had time to read all the posts but hopefully Im not repeating anyone, sorry if I am.

As a teacher I can tell you that as a parent you have a right to have access to the curriculum your son is being taught from. As part of your complaint ask the principal to provide you with information on exactly what subjects are being taught in religious studies, how they are being taught and by whom. Not all parents know/feel comfortable doing this but schools are accountable and it is your right to know this information.

Good luck I hope it all turns out ok.