I'll also pray that God might inspire you in your conversations with him, to find out why people withdraw from him. Is there a pattern in his conversation that gives them pause. Maybe he needs a little advice on social interaction. What does he bring to the relationship? What is he offering? Is he finding common ground? Does he have interests? Can he find groups that share his interests? Does he listen and encourage? Does he have a sense of humour and a spark to him? Does he talk about positive uplifting things or the negatives? Does he focus on self pity or gratitude?
I find that people who are upbeat, passionate about something in their life, and are good listeners, will find people who want to engage with them.
Does he know how to start and end a conversation or does he let them drag on, waiting for the other person to entertain him?
I occasionally meet people who want to phone all the time and don't know how to say good bye. I have to say, "Well I'd better be going" and they are "Well call my back when you are finished. I'm bored." I have things to do and end up avoiding these people as their calls don't have a reason. It is OK if it is a funny story etc. or a quick check in but I'd rather read a book, write a story, go for a walk or even meet for tea.
Maybe your friend needs help to review how he interacts and presents himself so that he learns how to show himself as someone that adds to another person's life.
I wish him well and thank you for being a friend to him. You may end up being one of his answers to prayer if you can help him.
Good luck.