Why doesn't Apple unlock my MacBook Air?

Please refer to Turn off Activation Lock. That's what you or you son needs to do. You absolutely need to know his Apple ID credentials, which may also have been protected with two-factor authentication.

The links at the end of that page are designed to help those unable to remember their Apple ID or password:

If you forgot your Apple ID - Apple Support

If you forgot your Apple ID password - Apple Support

Why doesn't Apple unlock my MacBook Air?

The reason Apple cannot help is that they do not know that information. It is stored in encrypted form and therefore irretrievable.

You will need the purchase receipt showing the serial number - take that and the laptop with you to an open Apple store. As long as you can prove you bought it, they may be able to help.