Minecraft is certainly available for Mac! You can see this if you visit the download page for the game, and under the "Desktop" section, select "Mac".
Although there are version differences between the desktop version of Minecraft and the iPad version (which is called Minecraft Pocket Edition), the foundation of the game is identical. Multiplayer is certainly available for the desktop version of the game.
Note that your grandson will need a Minecraft account for the desktop version (I'm not sure how Pocket Edition works, nor if you are able to use the same account information for both versions).
He wants to play the same multiplayer minecraft on the iMAC
According to this forum:
- Minecraft (the java version) that runs in Windows 10 (or older versions of Windows, or on Macs) is not "cross-platform". It runs on
- Win10 ("Minecraft: Windows 10 Beta Edition") is "cross-platform". It runs on PC's, tablets, and smartphones that runs Windows 10.
Win10 is also compatible with PE. PE runs on Android and iOS tablets and smartphones.
Consoles (like the java version), are unique animals and can only play with themselves... Xbox (and PS4,
and Wii U) can't play with any other editions due to different world
sizes, etc. (in fact, Xbox 360's can't even play with Xbox Ones).
Until, and unless, the (newer) consoles get infinite worlds they can't
play with any of the other editions.
The multiplayer would be separate (independent of the Pocket Edition). Pocket Edition players will not be able to play with people on Macs - making the multiplayer completely separate.