What is the default password for AirPort Extreme?

"Is there a default password with which Airport Extremes are shipped?"

Yes, but this will only be useful to you if the AirPort Extreme is still using its original factory default settings. The default password is.....

public

Chances are though, that you have already set up the AirPort Extreme with your own personal settings and passwords, so knowing the default password will not likely be of much help to you at this time.

"I don't know it and neither does my son who installed the AirPort Extreme."

If you have forgotten the password that you assigned to the AirPort Extreme, you can use a Soft Reset to reset the password. Once you have done this, it might be a good idea to write down the password and keep it in a safe place for the future.

Follow the instructions in this Apple Support document for a Soft Reset.

How to reset your AirPort base station - Apple Supporthttps://support.apple.com/en-us/HT201945

I haven't used an AE but with other routers it is usually a long series of letters and numbers printed on the bottom or back.

There are other more complicated ways that would require that you still have the Mac that you used to set up the AirPort Extreme originally......and.....that you know the administrator password for the Mac.

If you can meet both of those requirements, post back for more instructions if you are interested in a way to try to retrieve the forgotten password from KeyChain Access on your Mac.