Sorry for this late response...

I hope he ended the school year well? On fiction, has he tried the Bob Pflugfelder "Nick and Tesla" series? It's middle school interest level, about grade 4 or 5 reading level, and features gifted twins who use science and inventions in their adventures. It also has DIY projects embedded in the book, with directions for building interesting little contraptions out of (mostly) household materials.

...pronounced like the long vowel and first letter of the alphabet...