Look for good things to tell him about yourself. Have you completed college or are you going? Tell him something interesting you've learned. Have you developed a hobby? Tell him how you enjoy it. Maybe some things that you are doing that sound like fun would be just the thing to help him to see that life is not over once you are away from the borg.
He is reaching out to you. You have a wonderful opportunity to have him in your life. Through him maybe other family members will soften. At any rate, he has the right to his beliefs, just as you do. Tread carefully. He has taken one step in the right direction, against their teachings, that of contacting you. You may have a chance as time goes by to feed him tiny bits of information heading him in the right direction. Don't be hasty.
Tell him how proud you are of him, also. He doesn't get much but guilt from the WTS as we all know.