My 3rd Grader just finished (and enjoyed) the Wild Robot trilogy. What’s next?

This series was at his sweet spot in terms of reading level. He could stand to be a little bit challenged for his next book, but I want reading for pleasure to be easy and pleasurable for him.

He loved mythology, and was thinking Percy Jackson, but don’t know if it’s too big a leap for him. Harry Potter was a bit much for him 6 months ago. Magic Treehouse was awesome but too easy and not exciting enough. We’d love to hear suggestions, and apologies if I chose the wrong sub!