I installed pokemon go on my phone for my son. It's logged in with my Gmail account. I have now bought my son his own phone. Is there a way to log into the game using my Gmail account without all my emails etc. showing on his phone? I never set up a separate username account and have only ever logged in using Gmail. He obviously doesn't want to start at the beginning again.
Sorry Sir that is not possible. You will have to make a new account for your son.
Your e-mails should not show up on his phone if you log in to the game with your account. Pokemon GO only uses your basic profile info and e-mail address.
For example, my iPhone currently has two different Google accounts logged in to Ingress and Pokemon GO, and the mail app is connected to a third Google account. Android should be no different.
But if you still have doubts, you could always go the new account route.