What are some good ways to engage with the W40K lore?

I've never really played any games like this before but about a year ago my son came sprinting out of a toy store yelling DAD YOU GOT TO COME SEE THIS! IT'S THE COOLEST THING EVER! WHAT. IS. IT?! and now we sit here the two of us painting Warhammer minis and playing basic ruleset games together.

Since then, he has picked up some lore here and there, and we watched the Astartes thing on YouTube as well as that one episode that Amazon made, and I started playing through the Space Marine II game just to try to get some context myself so I could share it with him.

Where could we go now? He's old enough to read the core books but not full novels written for adults. I read for him every night and we could slowly work through those too I think, if there are really great ones, but comic style books or shorter texts are easier for him to take in on his own.

We look forward to W40K Total War but it's ways out it seems. I'm still on the fence whether I should let him play Space Marine, I want to at least finish it myself first. He can handle that type of gore and monsters, but not people being tortured or really vulnerable/helpless.

All ideas are much appreciated! Apologies if this has already been asked a thousand times, I tried to look through the Beginners Guide and subreddit rules.

PS. He's really into Ultramarines and I'm Imperial Fists. Painting yellow as a beginner is an experience.

EDIT: When posting I saw the links in the automod reply recommending a specific youtube channel and a lore wiki. Thank you! We'll definitely plow through that! If there are suggestions of stories taking place in this universe that is much welcome, too.