How do you communicate with your estranged son?
Well, you need to ask yourself why your son is no longer talking to you. Whether you think you did or not, your son most likely believes that you have wronged him or perhaps that you're judgmental or critical of his life.
Think objectively about what happened right before your son stopped talking to you. Was there a fight? Did you say some things you shouldn't have? If so, and you want to have a relationship with him, you should probably write a letter apologizing to him.
If your son has become estranged due to risky behaviors like drug use you may want to write and let him know that you still love him and don't want to judge him, just offer your support and help. Let him know that you're willing to get back into contact with him whenever he feels ready.
The reason why I recommend writing instead of calling or seeing him in person is because with a letter you can carefully plan out your words.