Welcome! If you have the emotional capacity to happily tolerate being disagreed with or ignored, you should absolutely participate in discussions. In the best case, you teach others something they didn’t know before, or get a misconception of your own corrected. In the worst case, your remarks are downvoted or ignored.
Your question on games would do well fleshed out into at least a quick take, if not a whole post, answering:
What games you’ve ruled out for this and why
what games in other genres you’ve found to capture the “truly simulation-like” aspect that you’re seeking
examples of game experiences that you experience as narrative railroading
examples of ways that games that get mostly there do a “hard science/AI/transhumanist theme” in the way that you’re looking for
perhaps what you get from it being a game that you miss if it’s a book, movie, or show?
If you’ve tried a lot of things and disliked most, then good clear descriptions of what you dislike about them can actually function as helpful positive recommendations for people with different preferences.
More concrete than your actual question, but there’s a couple options you can take:
acknowledge that there’s a form of social truth whereby the things people insist upon believing are functionally true. For instance, there may be no absolute moral value to criticism of a particular leader, but in certain countries the social system creates a very unambiguous negative value to it. Stick to the observable—if he does an experiment, replicate that experiment for yourself and share the results. If you get different results, examine why. IMO, attempting in good faith to replicate whatever experiments have convinced him that the world works differently from how he previously thought would be the best steelman for someone framing religion as rationalism.
There is of course the “which bible?” question. Irrefutable proof of the veracity of the old testament, if someone had it, wouldn’t answer the question of which modern religion incorporating it is “most correct”.
It’s entirely valid and consistent with rationalism to have the personal preference to not accept any document as fully and literally true. If you can gently find out how he handles the internal contradictions (https://en.wikipedia.org/wiki/Internal_consistency_of_the_Bible), you’ve got a ready-made argument for taking some things figuratively.
And as unsolicited social advice, distinct from the questions of rationalism—don’t strawman him into someone who criticizes your atheism until he as an actual human tells you what if any actual critiques he has. That’s not nice. What is nice is to frame it as a harm reduction option, because organized religion can be great for some people with mental health struggles, and tell him the truth about what you see in his current behavior that you like and support. For instance if his church gets him more involved with the community, or encourages him to do more healthy behaviors or less unhealthy ones, maintain common ground by endorsing the outcomes of his beliefs rather than endorsing the beliefs themselves.