Just get him the Airplane Flying Handbook
https://www.faa.gov/regulations_poli...lane_handbook/
Pilot's Handbook of Aeronautical Knowledge
https://www.faa.gov/regulations_poli...aviation/phak/
and Airplane Flying Handbook
https://www.faa.gov/regulations_poli...lane_handbook/
These are all free.
If he's still interested after reading that, get him one one the commercially available Private Pilot Test preps and sign him up for a written exam.
https://www.amazon.com/Private-Pilot.../dp/1619545209