So, i'm pretty sure, after the creation of the account, Google logged you in automatically, or you logged in to test the account and did not log out.

When your son starts Chrome on his Android device he is logged in by default.

I just want to make clear, that the reason why this happens is not based on IP, it's based on cookies. So when using services like Google or FB, never forget to log out, it can have really ugly side-effects.

And in general: setting the browser to delete all cookies, when closing is a good advice, i think.