Do you mean he used your pet name as part of the e-mail address or as the “from” name that is identified with your e-mail? In other words, I have an e-mail address that is email@example.com, but my name as it appears is abby stokes.
To change your account information in Yahoo:
1) Click on Options in the top right corner
2) Click on Mail Options
3) Click on Accounts in the left hand sidebar
4) Click Add or Edit Account at the bottom of the center text
5) Click Edit
Here is where you can change your “From Name”
Be sure after you make the change, that you click Save. Then send yourself an e-mail and see if the change has happened.
Let me know if that works.