Are you asking how to actually play together or just how to play the game in general?
There are many youtubers worth watching for ideas of what to do and also how to do things. I'm sure there are people on this forum who can give a more helpful answer, it's a pretty good community on here.
Ah lol. Well, I thought I'd teach my son to play games when he was younger until I realised that he was faster at learning them than I was!
Well, in creative you can do pretty much anything you want. Is there something he wants to build, like a house for instance? Just play around with different blocks until you find something you like. Again, there are loads of great YouTube videos you could look at for ideas.
You can also play survival mode, where you have to actually gather resources by mining for them. You can also set the game mode to peaceful so that no hostile mobs spawn until you get used to the game.
Nobody's really answered the question literally. To create a Creative world, you start a new file, choose creative as the filetype, and look over the other settings before you confirm world creation. You can either do superflat where it's an infinite flat field of grass, or a normal world generation, which is what I'd recommend. His little toddler-eyes will fill with wonder every time he finds a new biome.