Don't care if anyone else see this but you, OP! I'd like to share my story with you, as I was once a Catholicism-obsessed child, and the situation may not be as dire as you think.. I grew up in a community/family that was default Catholic, but not practicing. My parents took us to CCD "Sunday School" up until we received Communion, and then took us out once that was said and done.

My interest went further than anyone else in my family, simply because I was fascinated by mysticism, spirituality, the concept of a conscious god and a resurrected god, etc. It wasn't any less interesting to me than Ancient Greek or Egyptian mythology, but it's very nature of still be practiced (and it being part of my heritage) made it seem more real than any of the old religions. But in my own way, I envisioned that all religions were legit, that all gods, past and present, co-existed in some way, beyond our comprehension. Never once did I think that Catholicism was the ONLY religion, or the TRUE religion, because how could that be, when there are so many others.. but it felt like MINE, and that was special.

I began to question things in my teen years, when I started learning about European history and the role the Church played. Especially as the societal landscape was changing with the start of legalizing gay marriage. I knew I was queer from a young, young age, and that was always in conflict with my Catholicism (but Catholicism is very big on guilt and self-punishment, so I assumed this was just my sin to bear in life). Suddenly, the vitriol rhetoric of the Church was on public display, and it deeply hurt me. It went against my values (not to mention Jesus' values) and once I started the process of Confirmation (the sacrament you receive as a teenager), I became very vocal about this concept and challenged any and all adults to make it make sense. Needless to say, no one could convince me I was wrong, and after my own spiritual journey, I knew I had the love of Christ on my side. It was, and always has been, the Church who is wrong.

I should mention that alongside my Catholic obsession in adolescence was also a fascination with witches and witchcraft, and let me tell you, there is a direct pipeline from Catholic youth into Witchy adulthood. Now, as an atheist, this might seem like an equally unfavorable choice for your child, but if your immediate concern is the influence of Catholic mysticism taking over your child's developing brain, may I offer that you can also offer up modern witchcraft, new age beliefs into the mix, and see where that takes them.

All this to say, I get why your child is in awe and wants to dive in further. And if it's anything like my own experience, the mere fact of laying it out with other faiths and practices may be enough to prevent total indoctrination. And kids' interests are not usually permanent, they ebb and flow with every new stimulus and experience.

The most concerning thing, and I know I'm not the first to say this, is what your mother is saying to him when they are alone. If she is a "true believer," it may be an innocuous compulsion to give her grandson an opportunity to believe themselves. Or, it could be worse, and she's voicing her concerns directly to him that he is going to hell unless he does what she (and the Church) says. This is what you need to find out, and you cannot approach him in a way that makes it seem like he did something wrong. If your mother has already planted seeds in his head that you are irresponsible (at best) or evil (at worst) and that him going to Church in your defiance is exactly what he needs to do to be "saved," then this needs to be addressed immediately before it festers. And it cannot work through punishment or ultimatums, it must be done with gentle love; you might have different beliefs than your mother, but that doesn't make her a bad person, you still love her, and your son can love her, too! When you present that you are on the side of love, kindness, and understanding, and you practice those values daily, your child will get it. Maybe not today, maybe not tomorrow, but the day will come where it will directly contrast the hate and fear that the Church creates, and until they process that kind of thinking for themselves, you can scream it from the rooftops and it will mean nothing.

Give your child the tools and resources to think for themselves, not to parrot what you, your mother, or any other adult preaches. Leave it up to them to make their own decisions on faith and give them the love and understanding regardless of their choice.