The Prestons are all ghosts which means they all had to die around the same time and the fact that there's no mother with them could mean that she's still alive or something else happened to her.
The ways in which multiple people are able to die at once makes it even more disturbing, as it could be a multitude of horrific ways.
It is possible that ghosts reproduce asexually.
It has been mentioned there are 'ghosts schools' and 'ghost tutors' so it could be possible that the boy's mother is a ghost and they might be in the same situation as the Hathaways.
Or possibly child ghosts do not age past a certain age - probably the age of death - and such things exist as a substitute for stuff when they were living.
Here's a scary thought: As Black Nerd points out, the Prestons' clothes seem to indicate they died in a past era, roughly 1940's/1950's. The implications of what could kill a family of young black men in the pre-Civil Rights Movement American South are not very nice....