I would say that you must not think about programming itself. Programming
what?- should be your approach.
He should be interested in something, and then should face some problem in it, and then be able to solve it with programming. Or use programming to make things better.
I don't think that you can make a 9 yo interested in outputting a bunch of text into console.
You can get started with Scratch, and see how things go.
Or you can introduce him to Minecraft where he can get started with Java to do stuff in the game.
You can also buy Osmo and check how that goes. Someone I know teaches her 3 yo using Osmo.
My personal choice would be to buy a Raspberry Pi kit with some sensors or an Arduino kit, and download and use Open Source prewritten code with those. Then tweak the code to do things. Then eventually write his own code to do stuff.
Another way would be to introduce him to text-based games like the ones in early-years of computing. There's a book around called- Land of Lisp, where Lisp is taught with Text-Based Games.
I would ask you to remember three things-
1. There would be a trial and error component to it. You will have to be willing to try different things to see what sticks.
2. Code with your son. I mean, give him a place, a desk, maybe, to sit beside you and code together.
3. A boring thing becomes interesting when there is chance to outsmart one's peers. An interesting thing will become more interesting this way. So, after initiation, try and enroll your son in a group activity involving programming.
I hope this helps.
There was a Google doodle some years back where "programming" was introduced. You should check this out and give your son this [0].
[0]: https://g.co/kgs/NbCoHY