I was going to suggest the Wayside School books, despite them not being fantasy. I LOVED them as a kid.
I don't remember the reading level of this, but this was another book I loved in elementary school. It's not high fantasy, but it does have magic in it. Half Magic
Edit: there are also other similar magic-related books by Eager, if he tries that one and likes it.
Edit edit: Anything else I'm thinking of right now is too old for him. However, for when he is a bit older (I picked it up in 4th grade, but I was a strong reader so he might want to wait a little longer) I found Tamora Pierce, a wonderful fantasy young adult author. Most (not all!) of her main characters are girls, but that isn't a reason for a boy not to read them! I started with Sandry's Book, book 1 of the Circle of Magic.
Half Magic by Edward Eager has a magic coin and some time in Camelot. It was one of my favourites as a kid.
https://www.amazon.ca/Half-Magic-Edward-Eager/dp/0152020683
I started the Narnia Series around then and The Oz series (don't stop with just the Wonderful World).
Dragon's Milk (Involving girls and dragons and awesome.)
Finally, a classic I don't see mentioned much, Half-Magic, about a group of kids who find a magic token that grants half a wish. Hijinks ensue.
I was going to suggest the Wayside School books, despite them not being fantasy. I LOVED them as a kid.
I don't remember the reading level of this, but this was another book I loved in elementary school. It's not high fantasy, but it does have magic in it. Half Magic
Edit: there are also other similar magic-related books by Eager, if he tries that one and likes it.
Edit edit: Anything else I'm thinking of right now is too old for him. However, for when he is a bit older (I picked it up in 4th grade, but I was a strong reader so he might want to wait a little longer) I found Tamora Pierce, a wonderful fantasy young adult author. Most (not all!) of her main characters are girls, but that isn't a reason for a boy not to read them! I started with Sandry's Book, book 1 of the Circle of Magic.
That's the concept behind the book Half Magic. :)
Anybody else read Half Magic and its sequels? Don't know why, but they've always really stuck with me.