How do you get a son back who has banished you from his life?

Obtain a letter from him regarding the reasons and take it to your therapist. This will give you a third party opinion and you can work from there. Possibly short, friendly visits are the thing to do now without discussing his life decisions or anything else. You have to work through why he banished you in the first place. I dont know what it is but if you wronged him in some way you must admit that to him and ask him to give you another chance and go armed with solutions to the problem. If you did nothing to him and he is just indifferent to you there may be no solution to this problem. Good luck.