Reddit mentions of The Memory Tree

A couple other books to recommend for this age - Always Remember, The Memory Tree, The Invisible String, and Cry, Heart, But Never Break. There's a new one called Death is Stupid that might really resonate with him, but I'd preview it first. You can also try a kids grief activity book like Help Me Say Goodbye or When Someone Very Special Dies.

I'm so sorry for your loss.

When my mum was sick earlier this year, I started reading my 4 year old books about death and loss.
https://www.amazon.com/Memory-Tree-Britta-Teckentrup/dp/1408326345/
https://www.amazon.com/Ida-Always-Caron-Levis/dp/1481426400
These first 2 are beautiful stories about losing someone but keeping their memory alive.
https://www.amazon.com/Something-Very-Sad-Happened-Understanding/dp/1433822660
And this one is more direct and helped my son understand what was going on after she passed away.

Kids are surprisingly resilient and if you're open and honest with her she'll be ok.