If he had use your email account to create a new one then its simple to get access to that new account. First go to gmail and click on forgot password. Then you will have an option to send password on alternate account. Once you click on email password to alternate account, they will send you a mail in your email account. Now go to your email and click on the link that gmail will send you. Then change the password. Thats it. And dont forgot to change security question after you have changed the password.