This is such a sad story. Since he doesn't get much attention or is even groomed by anyone, he's probably turning into a feral or semi-feral cat. He may not be able to live entirely indoors either. I sure hope that your neighbors will take him with them when they move. If not then you'll need to help him find a new home. I bet he isn't neutered either and this is the reason why he roams around a lot and isn't seen for weeks at a time. What a sad lonely life for a cat to lead. Please keep us updated.