I'm filing my son's taxes for him. Does the email address for his account have to be his own email or can it be mine?

You can file for him using your account email! 

However, once he chooses to do his taxes on his own he will need to create a new account and input all his information from scratch.  Hope this helps!

Either option would work, but keep in mind that any important CRA notifications about the return will be sent to that email address.  Hope that helps!