I would start off with the basics.
Now I enjoy making games - but I do not use 'off the shelf' products. However, if one of my kids at this age suddenly became interested in making a game, I would not bother showing them any programming by default. What worked for me might not be the same for him.
I have never used Unity or the like myself so it is a good opportunity to do so. In your case, use this excitment with your son and allow him the power and opinions on which direction to go.
I would start off by suggesting that -
"there are 3 game engines I am aware of. Unity, Godot, and Unreal. Shall we spend this afternoon looking at them, installing them, and figure out what is best to use?"
This on it's own is not just an afternoon. It is (atleast) a weeks worth of content. Not only are you deciding which one to use, you are also looking at various tutorial websites.. likely how to "move things around" or setting 1st person or 3rd person perspctives, keyboard inputs, etc. Your son is learning a lot of things under the interest of his own game idea. In my opinion should keep him thrilled to continue.
Dont jump to the deep-end too early. He will likely loose interest.
In my world, I am interested in the logic of the game. Its the reason I became a programmer. However, and I think for most, when they are interested in making games -- it is about 'designing' a game. Coding is just one aspect, so find the areas that interest him the most when learning.
For all you know - this could be the starting point of him becoming a photoshop guy, or a blender guy, not just becoming a level editor guy.. or a programming guy.
Give him control and help guide him into the direction he wants to go. Maybe express interest doing the bit he isn't into doing just yet.
Just my 2 cents.
-- Edit: added some more content --
I became interested in "how does this work" when watching Street Fighter 2 for the first time.. along with many other games. It was not until I was 13 and seeing DooM for the first time I was like "I have to figure out how they do this!"
My era with these games, especially Quake with QuakeC, or Level Editors like Worldcraft, was my days of learning and figuring out games.
What I have learned is to not interject how I did things in the 90s when I was young compared to how the young can do it today.
At a simple level, they have Scratch. They have Minecraft. They have GMod. They also have Godot, Unreal and Unity.
Point I am making (and believe is the same you and your experience) -- avoid interfering too much. That would be my priority.