A good bet is alternative media, things like kurzgesagt, verisitium or other science outreach programs.

The other thing is education, engage with his education, and teach him yourself to think critically.

And I hate to say it to say it, but religion conditions kids to brainwashing. Even if not extremists.

Feel comfortable pointing out flaws in Christianity and question them with him.

And lastly, try not to be argumentitive. You need to carefully tease him out of these beliefs, not destroy them.

That would feel like you are attacking him and will probably be counterproductive.