What do I do if my ex-husband who has sole custody of my son whose 13 now, will not allow me to see my son whatsoever?

I don't know my ex-husband's address nor phone number, because he has never given me of their whereabouts ever. When I signed the custody papers back in 2004, I never even was told that I could get visitations nor got a copy nor my ex-husband ever call me to tell me. I found out last December that my ex-husband had moved out of state with our son. My ex-husband never told me that he was leaving my son's birth state to move to another state. I've waited almost 12 years to seek my ex-husband to see our son and start having a son-mom relationship with my son because, I've been in constant fear of my ex-husband. Only, because when he and I were married, it was an abusive marriage where he used to abuse me physically and emotionally. And, after the divorce, he and I did have joint custody. He bullied me for months to give him sole custody of my son to him. So, feeling powerless and defeated and the fear from him, I signed the custody papers giving him sole custody. I've missed my son so much and love my son so much in all of these years, that I finally got the courage to seek my ex-husband if he would allow me to see my son but he won't. I've contacted his family, but none of them will help me locate my ex-husband. I even contacted his new wife on Facebook to ask of her if she could talk to her husband who's my ex-husband if he would allow me to see my son, but she blocked me and I haven't heard anything from them. My ex-husband's brothers blocked me from their Facebook accounts when I messaged them about them talking to their brother about me seeing my son. I messaged my ex-husband's sister in-law but she blocked me too. It's like my ex-husband doesn't and won't allow me to see my son at all, keeping my son and I away from me.