Silence will fall.
I have told him that the Silence were made up for Doctor Who. I have told him how it all ends for the Silence, I have given him a pen and told him to make a mark if he sees the Silence, so he can always look at his clean, mark-less arms and see that he hasn't, and I have made the logical argument that, if the Silence were real, he would not be able to remember seeing them on TV.
What else can I do to help him through this fear? I feel horrible for forgetting the Silence were in that episode.