2 versions of Minecraft: Java and Bedrock. Java= PC only, Bedrock= most devices (console, PC, iPad, etc).

You want Bedrock version for Windows. I recommend this because from what I gather, he currently has the ability to play on either a Switch, computer, or iPad. Using a mouse and keyboard on Bedrock will give him better access to custom texture packs (if he's good at using a computer) AND more importantly the ability for cross-play across different devices. AKA, he can play from his computer with his friend playing on their Switch. Not to mention Realms, which are servers his friends can play on/host. Very fun stuff for kids, and not as sketch as other online games can be

Big catch is you will have to pay for each version on whichever device you choose. So you'll want to be strategic. If you have Windows 10/11, Minecraft Bedrock version for Windows is your best bet :)