Oh okay, I thought you said that he was still in the detail screen.. he clearly did not leave for college because of his age and you'd get a notification, same thing if he passed away.
It could've been the talent scout event and some sort of bug related to it as well as all of the other events that would remove a child from his/her home.. just trying to think of reasons a child would disappear randomly and without notification.
Sometimes new children or husbands/wives will also disappear if your game experiences a crash or something happens to the save files which is why a lot of people save backups in other locations of their computers.
Sorry you lost him, but as long as his picture isn't dimmer than the other kids or adults on the family tree, that means he is still alive and you can choose him to inherit the house if you wish as Lewanda has mentioned.
It would also help to mention what version of the game you have as well as where you bought it from so we can keep a look out for this bug.

If you're not sure, you can go to the main menu on the bottom-left corner and it'll have the version there along with letters representing where you bought it from.