How do I open a second account for Minecraft?

"Accounts" in Bedrock Edition work differently to Java Edition. In Java Edition, it is both your entitlement and your profile. However, in Bedrock Edition, you can simply just switch Xbox Accounts and your game will use a different set of player data. Your entitlement (Google Play/App Store/PlayStation Network/Microsoft/Nintendo account) is seperate to your Xbox Account*.

As for playing together, you just need to redownload on a second device using the same Google Play/App Store/PlayStation Network/Microsoft/Nintendo account and redownload the game and make sure you're logged into two different Xbox Accounts.

*You log into an Xbox Account with a Microsoft Account, but in terms of how entitlements work, they're separate. On Console (Xbox/PlayStation/Switch) platforms, each player needs to own their own copy, so you have to log out of the account completely on your consoles in order to get the game working.

Also entitlement = purchase of game aka. right to download

You did specify you are on PE, so you are likely on a Mobile (at least that's what I think).

If you want to make a 2nd account for your grandson, then try logging out of the existing account and create a new one with another e-mail address. Then after some time, you can customize the account's items.

Funny thing is that the worlds still appear even if you switch accounts. That's because all the worlds aren't accounted by the XBox accounts in the game, but rather from the External and Application settings for world access.

You said:

My problem is every time I go to purchase the Minecraft again it sends me to the game I already have.

Isn't that how things really are? If it is the same device, then it will bring you to that same game, then bring you to all the stuff you have when you didn't have an account yet, because you re-purchased the app, your account isn't logged in.

You have to be in another device to actually purchase the game with default settings, then you can set up an existing account there if you already made one on the previous device.

It doesn't matter where you play it, as long as you are still on Bedrock Edition, you can play multiplayer together.