My son has signed a birth certificate he has always been her father no matter what he was not married to the mother at the time of her birth. But did get married 2years later.Had a son lost him at 7 weeks and 6 mons, later got divorced.The little girl is now 5 and he is still in her life as her dad. Now her mother has came to him and asked if he wants to adopt her. Well sure he dose he was ordered to pay child support though the divorce. But that's when he find out after DNA Test that she was not his. Judge stopped child support. But he see her all the time and takes care of what she needs. Can he adopt her she has his last name

Since he is not married to the mother now, this may be difficult. Also, since DNA shows he isn't the father the bio father will need to be contacted for approval.

But given his relationship with the girl, a court MAY grant the adoption. The only way he will know is if he gets an attorney to file the petition for adoption.

IF he was already on the birth certificate there was no requirement to "adopt" he was already the legal father.

I only see a DNA test as a method to take him off and take away his rights as a legal father.
Why and who asked for a DNA test ? That was the last thing he wanted if no one was contesting his right to be the father.

No he can not adopt the child. He has to be married and/or in a relationship with the mother,

Who has changed the birth certificate taking him off ?