It is really loving of you to try to understand his illness.
May I ask? You mentioned that you have been in a long distance relationship. When was the last time you saw him and how often did you see him? I am asking to understand the depth and intamacy of your relationship; because, like Seesaw said, I do not understand how he just ‘disappeared’ on you. I understand withdrawal and isolation, I do not get why one would ‘disappear’.
I sincerely hope all works out for you.