If he was younger, I would have recommended scratch. I recommended that to my then 11-year old cousin and he is now an engineering undergrad student.

At your son's age, there are plenty of how to program python courses using udemy and the like that show to build progressively harder apps for a complete beginner. From there, he could probably try to do one of those google certificate courses on android development or the like.

I remember having to check out BASIC programming books from the library as a kid to learn. These days, people have made fortunes on creating videos on how to program.