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 know I need his permission but he won't sign. My question is is it likely for the judge to agree with me and hyphenate it

Very doubtful but you can try, There is no real "need" for the change, just desire for the change.

'After he is served with papers.. ' - what papers?
I don't think this can happen.