So, sperm donor (SD):
1. not on the birth cert
2. never married to mom
3. never provided support for the child
Why does the kid have his last name? Was that last name given at birth even though SD not listed as father? Basically, how is he legally recognized as the bio dad other than your wife "mistakenly" thinking so?
Is there an end-around to somehow legally acknowledge that the SD is NOT the father and then it negates the requirement to have SD "sign-off" on name change? If SD somehow proves (DNA test) that he is the father, hit him with 'child support' demand. Does he have an ability to pay for years of child support? If he crawfishes, like the cockmunch he sounds like, and disappears then you get what you want.
Obviously I'm not a member of the Surly Legal Cartel, nor did I sleep at a Holiday Inn Express last night.