I struggle with this as well — the desire to overly curate my children’s lives by funneling books I consider “valuable” into them like a kale smoothie. There are times I curse the day we ever let the fast-food burgers of Big Nate or Captain Underpants into this house. We’re almost through this phase (three of the four have grown up and gone off into the world reading good books), but the littlest one is near 12 and mostly wants to read graphic novels and Manga (which wouldn’t be so bad if he would read the ones I want him to…. But I’ll be damned if these genetic vessels of mine didn’t turn out to be willful little shits like their dad.)
Aside from having to do some parenting around Manga’s gratuitous violence, body image, visual sexual innuendos, and, frankly, consent (get it together Japan), I also suffer from a some sort of instinctual disapproval of the idea that passing your eyes over these books constitutes “real reading”. But having been through this before, the fourth kid is getting the benefit of my accrued wisdom (and exhaustion) and is largely being left to his own devices when choosing books.
Don’t get me wrong, I still give him books I think he might like, but I try to not pressure him into reading them anymore. Won’t work. I’m just happy he’s getting narrative into his head. The best thing we can do is model reading. Let him see us reading, tell him about what we’re reading, read in the living room with him, etc.
When a child finishes a book they love, they don’t see how challenging it was, they see that they finished it and all of the pride which comes with that. On the other hand, when they attempt a book which doesn’t engage them or that they find too challenging to finish, they will feel that strongly and you run the risk of turning them off trying again with something else.
You should also not discount how important engaging with art is for their cultural development. Illustrations offer a huge amount in the way of context for the story as well as just being something that should be enjoyed in its own right!
But at the end of the day, none of this really answers your original question. My advice to you is twofold. First, try to take what I’ve said above on board and by changing your attitude towards the books your son is choosing you might lessen the anxiety it is causing you. And second, if you still want to try and move them on, baby steps are the way to go.