Why book? I don't know about you or your cousin, but learning from a book is so slow and dry. Why not a video series or online tutorial? Hell, maybe teach him the basics and build a small project with him.

Codecademy is a great way to learn a bit of syntax and then build something. 'How to make a website' is a good intro course for getting into web dev.

There's also a lot of good python basic videos on YouTube, then he could dive into ML/AI with:

Machine Learning Recipes from Josh Gordon at Google https://www.youtube.com/watch?v=cKxRvEZd3Mw

Siraj Raval's channel on ML/AI/Programming https://www.youtube.com/channel/UCWN3xxRkmTPmbKwht9FuE5A/vid...

All that said, if you reallllly want a book: "Learn Python the hard way" by Zed Shaw https://learnpythonthehardway.org/