what is the best way to run games on a IMac desktop

Too open-ended. There are games that run on Macs, there are games that run on Windows, there are games that run on both. There are also online games that run in a browser. The games he wants to run will make a difference.

The other missing link is the variant of iMac. Entry-level iMacs often have the most basic video hardware that won't support graphics-intense games and sims. It also may not have shipped with enough RAM to meet the game developers' requirements. Older models may not be able to run a new enough Mac OS version.

Your grandson needs to do "About this Mac..." from the "Apple menu" at the left end of his iMacs menu bar to get this:

Important things that screen will display that we need to know if we are going to give effective help are:

Sub-model

OS version

Memory/RAM

Video hardware

Note that I've redacted the serial number. For security reasons please do not post a serial number in the forums.