I would try sending your friend a letter, snail mail or e-mail, and tell him that being isolated is psychologically unhealthy, and that you're worried about him. Tell him that you care about him and would like to see him in person somewhere. Try to build a relationship with him slowly. You might try showing him psychological research that shows being alone to be unhealthy. Maybe give him something to read about Avoidant Personality Disorder, or the psychology of Sullivan or Erikson. You should realize that this type of personality might take a lifetime to overcome.