Mojave to Sonoma?

Yes. All of the apps included with the OS will already be 64 bit.

An example of a 32 bit app that will no longer work would be Microsoft Office 2011. This also includes The Adobe CS6 suite and older. Even though the CS6 apps are 64 bit, they also use 32 bit dependencies, so they still won't run.

You can use this free app to check and see what apps you have that may not run in Catalina or newer. Or, you can check yourself using the OS to look for you.

Hold down the Option key and click on the Apple logo at the top left. Choose System Information. Click on the Applications heading in the left column as shown below. Expand the right window and adjust the column widths as necessary to see the Kind column. Click on Kind to sort by CPU/bit type.

Below is on my M2 mini, so it reports the one 32 bit app it found as unsupported. On the 2018 Mac, 32 bit apps will simply say, 32-bit. Any of those will not run in Sonoma.

As long as his MacBook Air supports Sonoma, then yes.

BUT!, be aware any 32 bit apps will no longer work.