Early adolescence books with boy girl friendships

Our 13 year old grandson, a grade 6 student in a Waldorf school, is now interested in fiction with human characters. Previously he has preferred books like Cricket in Time Square and The Rats of Nihm.

We are currently reading Kate DiCamillo's The Tiger Rising and he is captivated by Rob and Sistine's friendship.

I would appreciate recommendations of good books with similiar fruendships of boy girls age 10 to 13. He's not yet interested in high school dating relationships.

Thanks