I just found out I have a twenty year old stepbrother, what should I do?

I got a really long message on facebook from this guy saying he is my stepbrother, and he has been wanting to tell me or a long time. My dad has never told me about it, and all of my relatives on my dad's side know him. He wanted to meet me when I was visiting my relatives on the east coast, but he said my dad wouldn't let that happen. I do not want to confront my dad. He also said that my dad stopped raising him when he was 10, because my dad apparently wanted to start a real family. What should I do, and what should I say?