Not something I am terribly good at but maintain the contact. You don’t have to have fancy words, you just have to be there. He may not always respond, but knowing you are there, even in the distance has to help. I think I remember the friend you are talking about from an earlier conversation.
Often, when we go through traumatic events people are there initially but then life takes over and we can end up alone. You be one of those people who doesn’t drop off. Do as @fremen_warrior suggests and set up a ‘skype’ date and have a talk with each other each week at the same time.
I think normalcy is the key. While there may be times when he needs you to mentally hug him, I suspect he also just wants people to treat him in the same way as they did before he went through all these problems. Share positive things you are doing in your life, books you have read, films you have seen and the like. You know this person, what has been his passion – talk about that. And as @Mariah said, when he needs it just be quiet and listen.
Talk about the things he has always loved and just be there.