Wow- this is a tough one. It's disheartening to hear that your child is using the new husband's last name, when you are still the biological father (and I'm assuming scheduling visits and paying child support.)  I understand your frustration.

I can see, though, why your ex-wife would want to use her new husband's last name with your son, because then the faculty and school know that she is the mother of your son. It's good that she is proud of him and wants to be associated, at school, with her son.

But- I agree with you- unless they have changed his name legally, your son should be called by his correct last name. Call the school principal or vice principal and state your concern. Inform them of your son's correct legal last name and ask for him to be enrolled and addressed by that name. Also, while you are talking with them, make sure they have your correct contact information.

Ask for a school calendar so you can keep up with special events, visit the school day, and parent/teacher meeting days.  

Blended families are tough, but you are still his father and have a right to be active in his life, and for him to carry your name. Don't be discouraged, be proactive and contact the school in your friendliest and cooperative voice- and I'm sure you will be able to get things changed.

Shirley Cress Dudley, MA LPC
Just Ask Blended Family Expert
Director of The Blended and Step Family Resource Center
http://www.BlendedFamilyAdvice.com