Dust off your crystal ball and GUESS the horsepower !
Get him this book for bathroom reading so he can nerd out. It will probably help him hold interest over time because he will have a good understanding of the mechanics. Who knows, he may want to become an engineer.
Do the math with him on each part of planning the build, so he gets a good feel for WHY he has to learn math in school.
https://www.amazon.com/Performance-Automotive-Engine-Math-Design-Pro/dp/1934709476/?tag=fabo03-20