Lifecraft is an evangelical atheist so ignore.
Perhaps try and find some good childrens resources online. He might enjoy the bible project.
Bible stories are interesting regardless of faith.
I think churches are quite variable on their relevance to children. Tend to bore them solid (that might be mine and my dc experience). It can't hurt to take him.
If you are going to go to mass it would be a good idea to read up first. Even just a Wikipedia entry on it or theres probably tonnes of "what happens at Catholic mass" explainers online. I think theres a lot of standing up sitting down saying things from mass sheets and hymn books it makes me anxious and I'm an insider!!
You can get children's missals on Amazon cheaply. They talk about mass and the different prayers etc. The good news is once you get the hang of it its the same regardless of place.
Being a Catholic means being baptised then eventually taking communion. Id gently explain that you can't just rock up and be Catholic you need to learn about the faith.
Dont get me wrong Im all for spirituality and I think belief is really important dimension to life. The real positive here is your ds is attempting to communicate with God. That can be a search lots of adults haven't yet started on! Is he a very mature child?
Christian beliefs can be very positive to have a framework for thinking of and helping others.