How can I obtain sole legal custody of my son if I don't know where his father is?

His father has gone several years at a time without seeing him and my son is now 15 years old. I want to take him on vacation to Europe next summer but I can't even get him a passport because I can't track down his father to notarize something so I can. I have been in college for several years and just have not been able to afford a lawyer to handle this for me. How do you prove that someone has been absent, besides 2 visits, in 12 years? The last time he came to visit he went to the courthouse with me to give up his parental rights so that we wouldn't have to worry about finding him for major decisions, but we were told that only the state could petition to dissolve a parent's rights. I haven't been able to find any information specific to our situation. Please help.