I think he mixed up answers Smegs had made him memorise. Charles's condition was classified, and what Farkie did to make him laugh would have been between Sparry and his "son".You know he is lying when he says he'll keep any details about his father between them both. As if Charles actually told that hole what the diagnosis was.
This is what happens when you learn by rote rather than understanding what you're reading/hearing
The peacocks in the forest opposite the road are all screaming happily I love these guys, their screams bring me contentment