Son with serious depression

My 30 year old son has suffered serious social anxiety and depression since the age of 15. He takes antidepressants but has always refused to seek other help or talk to anyone. He works, but hates working, lives alone in a small flat and has few friends. He's a skateboarder but has only acquaintances through that. I live an hour away in the country. He has recently begun calling me from work in a state of high anxiety, saying he just can't cope, occasionally has suicidal thoughts but would never act on them. I race down to see him and we talk. He refuses to get professional help, is afraid of going off his medication or changing it, will only talk to me. He won't come to stay with me or even visit. He has had no contact with his father since he left the family home (when my son was 15) and his sister lives overseas. I worry that he is so alone but I just don't know how to help him. I would appreciate any thoughts or contact with someone who is dealing with a similar situation.