BBM:
I'm a big believer in this theory.
It fits the facts we know to date.
There's nothing we know that contradicts that theory, either.
I firmly believe he was injured/harmed somehow on Sunday, and that his condition worsened.
I also believe that he was harmed in such a way that there was no plausible alternative explanation to explain away his injuries, i.e., "he fell down the stairs," "he bumped into a door," etc.
He may also have threatened to tell someone.
At which point, his phone would have been taken away from him, to ensure he was unable to contact one of his parents, dial 911, etc.
JMO.