How do I change my son last mame he has his mother last name my name in not in the birth certificate after he born she got married with an other boy so he has that man last name I did a dna test can that help me

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

You need to go to Family Court and file that you be named the legal father. Petition to have the birth certificate amended.

How old is the child? Did the mother marry the other man before the child was born? Where are you located (state, country)?