Csab, im sorry to say that i think that he is not being honest with you either.
I didnt think of the phone thing, but there is no way that if he was scared he would take the time to roll up stuff to make it look like he was in bed - especially as he would be seen, spirit or otherwise (and im sure he knows that you would believe him re the spirit stuff as this is something not uncommon to you)

I like the idea of making the window squeeky or hang a bell off it or something similar, but i do think that hiding a monitor in his room is an invasion of his privacy, he is a 16yo boy after all and there are things that he may get up to that are harmless but embarressing to him ... not to mention the fact that it would damage trust between you if he ever found out.

Sorry that you have had a rotton night of worry and then being hurt in the morning