My sons father sees him by choice 10 days a year. I gave myson his fathers last name under false pretenses that he would be an active part of my sons life. My son is 5 and doesn't like the fact that he is the only one with a different last name in the house although he knows where his last name came from. I asked his father about signing to hyphenate it with both last names so my son doesn't feel left out but of course he won't sign. After he is served with papers is it likely for the court to agree to hyphenate it since I am not changing it from his fathers just adding mine?

I don't know where you are BUT in 99.9% of the time you need the permission of the father to change the child's last name.

You need to check with your local Clerk's office.