I am not on my sons birth certifacte and want to prove I am the fahter how do I do that

You and your son submit to a DNA test. You can do this on your own by sending in swabs to a lab (check for some online), or you petition Family Court if the child is in the care of someone who isn't willing to do this voluntarily. Keep in mind that the court will most likely order monthly support payments for the child if you are the bio father.

Why aren't you on the because? Did you sign an Acknowledgement of Paternity. ANY question on law needs to include your general locale as laws vary by area.

Why do you need to prove it? Does the mother deny it? Do you just want to prove it to yourself?

As you can see your brief question leaves us with more questions so we can give you a proper answer.