First thing you need to is to make sure someone didn't accidentally or purposely delete the account from the Users section in Control Panel. When a user is deleted then it asks if you want to delete the users files as well.
If not then you can look to see if the C:\Users\username folder is there (username will be your sons login name). Open Windows Explorer and browse to the C:\Users\ folder and see if your sons name is there and if there is anything in the folder.
You can also do a search of the entire computer for his files in case they were moved if you know the names of some of them or you can do a search for his username.