Thank you for using Apple Support Communities! I understand that you would like to stop your sons photos from importing to your MacBook Air. I would be happy to help.
The first step would be to determine how the photos are being imported. If this happens when his device isn't connected to your MacBook Air, it is likely a results of iCloud Photos:
Set up and use iCloud Photos - Apple Support
You'll want to confirm that you are not sharing an Apple ID for iCloud on the MacBook Air and his device. You'll find this setting on your MacBook Air by going to System Preferences > iCloud. You can also uncheck Photos from this screen to stop syncing iCloud Photos between devices. It's also important to keep in mind that if iCloud Photos is turned on, any photos deleted from either device will delete the photos from iCloud Photos as well.
The scenario is that the photos are being imported manually when his device is connected to the MacBook Air:
Import photos from a camera or phone using Photos on Mac - Apple Support
If this is happening, and iCloud Photos is turned off, it is safe to delete the photos from your MacBook Air without having them erased from his device as well.
Cheers!