I am in California, I am curious on how much it will cost and how to add another last name to my son's name where he has both mine and my ex last name. My son is 10 1/2 months old

You can't change his name without the signed consent of his father.

Here is info on fees and how to get them waived: California Courts: Self-Help Center: More Topics: How to Change Your Name: Forms for Use in Name Change and/or Gender Change Cases

Even if I was to just add his last name... his last name isn't on the because... I wanted it on there but so many people we giving me such a hard time... I see that you are from ny is that apply there or getting consent apply here in calfornia

Judy is right, in NY of CA, you need the consent of that father for a legal name change.

Did you read the site I posted which WAS California law?

Everywhere in the US you need the signed consent of the father unless there are unusual circumstances OR unless you have the time and money to petition the Court and ask for permission without the father's consent. I doubt it will be granted.

Who is giving you a "hard time" and for what reason? The law is what it is. There's nothing for anyone to argue about.