How do I change my child's last name in kentucky?

I live in western Kentucky and my kids are 12 and 9 and they haven't seen there biological father in 8 years. They wouldn't even know him if he ped them on the streets. My son came to me and asked me if I could change his last name to his step-fathers which is all he has known for the last 7 1/2 years. I am not worried about legal adoption, I would just like to give him what he wants a new last name that will be the same as ours. He even refuses to write his real last name on papers at school and his teachers get on to him, but he tells them that that isn't his last name. I can't afford an attorney, but I don't kow where to get started to do it myself. I have called the couthouse, but they tell me that I have to have permission even though he doesn't care about them and has been in and out of prison. I wouldn't even know where to begin looking for him. I need help. What do I do.