I've Been the step father of my son since he was 3 years old, he's now 15, I want to change his last name to match mine and his mothers, he's HAS never met his real father nor does he care to. We live in Washington state. He's harder to find then a needle in a haystack, has numerous kids, who also have no contact with him.

Why not just adopt him? That may be easier to get through the courts. Either way you will need to show an effort to contact the bio father.

You really need to discuss this with an attorney.