OK I think you can tackle this in steps
I have always read - and have large books full of images - graphic novels etc
I get books
But we also had minimum space when kids were young and of course they also needed books
It became easier to get rid of books once we had to do it for our kids
So start with novels he won’t read again - there is always new stuff coming out and if you won’t read a book again it’s just a dusty ornament, if you give it to charity someone else will give it new life and it will be read again
Your tastes change over time, what you read at 18 is likely to not be the same as what you read at 35
Your children will have their own tastes and whilst I can understand keeping some excellent examples of books (Hitchhikers Guide to the Galaxy for instance) there is no guarantee that your children will read the extensive collection of Iain M Banks books (random example you understand)
Two full bookcases is plenty for new parents in a small flat.
But start small - novels you won’t read again
The upside of decluttering books is you do read more. I had never heard of Fahrenheit 451 - supposed to be part of the holy trinity of modern dystopian novels, I’d read Brave New World, 1984 but not Fahrenheit - the teens have read it at school - so I am reading it now (I can see it appeals more to a younger mind) - so your future reading can be influenced by your children as well as the other way round.
It’s reading that’s important not keeping hold of everything you’ve read. And as I get older I realise the number of books I have time left to read is shrinking - so they need to be worth it and as good as Bill Bryson is (and he is good) I won’t re-read his books so they go (and neither the teens or H would read “At Home” I do check)
Graphic novel Cassandra Drake is brilliant as far as I am concerned - great way of storytelling - brilliant story, youngster took that one.
So we are a family of readers, we talk about and can be influenced by each other’s books, but still donate ones that have served their purpose for us and move on to new ones.
It might take months but a book declutter helps. And allows space for new books and new stories.
As for non-fiction, because we read that as well, some books become irrelevant on topics or at least not as informed as time goes by, or rewritten in more user friendly terms (String Theory for example). So you can apply different rules to them.
I think it’s important to have non-negotiables if anyone tried to take my Edward Hopper books away I would bite them. But I reference them all the time (and drag the family across the world to see the originals) they stay!
Good luck @wiredandtired