Where is he at school where he is having dedicated RE lessons in first year school?

TBH, given your reaction, I would pull him out of the classes. I am not Christian, nor am I agnostic though I do not follow a monotheistic religion, and my children go to a public school, and they have never even come close to having an RE lesson or anything that could be construed as one. This baffles me. I didn't even do RE at primary school until Grade 3 - and there were many people who opted out.

I digress, Explaining the concepts of God and religion are not so black and white as "some people do and some people don't" at this age. They ask all kinds of questions in an effort to understand an abstract concept like "church" or "god" or even what the Bible actually is. So it is really hard to address these issues in a manner they can grasp.
I do think though that you personally have quite an issue with the conviction with which he is being taught - Religious Education class should not be confused with Religious Instruction - which is what this sounds like. And 5 and 6 year olds and really too young to be looking at the vast array of religions out there.

My children would not be participating in a Christian Instruction class at this age, for any reason.