I think this is a reasonable possibility because if his mother or relative was living up there, he would at least tell them that he's not coming back in Bright as Day. He just decided he would stay and didn't seem too sad that he isn't going back, seems like he has no relation ties to anyone on the surface. If he did have a mom up there, he would at least say good-bye. It also makes sense since when his father disappeared, it seems no one in the neighborhood tried to look for either Will Shane and Eli was still living in the same house. So did Eli live alone for five years after his father's disapperance?