Now that I have discovered that Project Gutenburg have free Kindle-compatible books, I am rereading some old favourites. Currently reading "The Secret Garden" just finished "Kim" and have "The Jungle Book" downloaded ready. All these might possibly suit your grandson, JingleJ ? and free so having a look won't cost anything. found "The Water Babies" a lot longer and more sermonising than I remembered, maybe i read a shortened version last time (a long time ago)
They have a lot of PG Wodehouse, but the Jeeves and Wooster ones get a bit repetitive read one after the other. I preferred "Love among the Chickens" which is not J and W. but a nice gentle love story, with Wodehousian humour.