Jerry has been mute since his father died. This may be a side effect of the trauma, or it may have been malicious and intentional. Perhaps he saw something he shouldn’t have and the timing was good enough to take away his speech through supernatural means.
Either way it was a big enough deal for Master Paine to open up a can of Pandora’s whoop ass to try and help Jerry. It didn’t turn out so well.
Jerry spent about a day ‘auto-writing’ all sorts of Enochian script and drawings. Master Paine said “Jerry is the Key.”