Pokemon-go – Changed email address

If you're using a trainer account you can do so on the Pokémon website. With google the Pokémon account is linked to that account, so you would have to give her access to it.

If you are able to sign into the app, you can go into settings and logout of that account to log into another one. You will lose your account data doing this. If you cannot log into the app, and get stuck at the loading screen (with the Gyrados) there is a chance for it to have a "Sign Out" button right in the center. That will allow you to get out and sign in with an email address/account you control.

The problem is that your son effectively created his account under someone else's name, and now that account is theirs and he will not be able to get it back. You'll need to create a new account, tied to an email you already have. Either your son will need to remember his email account or create a new one before signing into it in the game. Unfortunately he has no other options.