Part of what is going on is probably survival mechanism. You have to question, does child feel safe to disagree at other house? Because that might be the cause of the entrenched views. Kid can't question the beliefs, even away from there, because they would smell the doubt and it's harder for kids to hold one belief and lie. Just make your home a safe space, and ask questions, so many questions. Let his mind hold those questions in the background and one day they'll unlock his mind from that brainwashing. Just build him up, make your home a safe space for anything. Question your own beliefs, don't just do that to his, ask questions about everything. Be open to saying this is just a belief, not fact. And that it's still OK to believe, but knowing the difference between belief and fact and they can both exist. Don't challenge the individual thoughts, teach him how to critically evaluate something, don't just give him a fish, teach him how to fish. I wouldn't challenge his beliefs at all, I would focus on other things and question those, avoid conclusions. Let those be unsaid, for now, or let him come to them. If he is criticising your beliefs, (saying you believe in science too much) you are a safe person. Just make sure they don't reel him over there full time, keep your time, maybe even try and get an extra day or two

Why do all the scientists lie? What do they get? How do they get convinced to lie? Research who pays them, if it's money. Ask as if you want to know, not to debunk. Be interested. Which sciences are fake? How do you tell which are fake? Who decides what's true and fake? If you delve deeper into any of the things they don't hold water, but don't say that. Just let that uncover itself. Make space for him to have his beliefs and respect them. Arguing for him to abandon his beliefs will just push him away.