Start with Tetris when he's 4-5 years old. It's an incredibly simple game with very simple controls. It has almost no text, no story, no suggestive imagery... Perfect for a little kid. It's literally an electronic puzle. With it he will learn the basics of gaming controllers and the art of pushing a button/performing an action on screen. And of course Tetris is great for the mind.
If he likes it and once he gets the hang of Tetris, try a simple 2D Platformer. Something like the original Super Mario Bros.: you walk/run right and left, you jump, you throw fire sometimes... and that's about it. Don't pick something with lots of power ups or with lots of mechanics like grabbing stuff or double jumping... The idea is that after learning to use a controller and to apply some logic with Tetris, he can try to navigate through a 2D World.
After that he will be able to try some slightly more complex games. When he learns how to read (more or less), you could give him something more text-heavy and intrincate like Pokémon. I think that franchise is perfect for introducing children to their first "adventure". Of course, he will get lost, he will struggle to learn some mechanics... but he will eventually get it. And of course you'll be there to guide him.
If he gets this far... then I would let him decide what he wants to play next (with limits, of course. Don't give him CoD for the love of God xD). You can make suggestions of course, but choosing by himself will develop his tastes. He doesn't have to like the same as his dad. Who knows, maybe he'll want to check a 3D game next. Whathever it is, keep playing with him. Maybe not all the time, but be there for him and have fun with him.
Whathever games you choose to use and whenever you end up introducing him to videogames, I encourage you to always play with him. Don't let him alone while playing, specially until he's like 9-10 years old. First because you have to make sure he doesn't play too much and because he'll need guidance both to understand how everything works and to not be alone if something weird appears on screen. And second because you gotta make him understand that videogames are something that can be enjoyed in company.Last edited by Vodacixi - on 26 April 2021