What does Scrooge learn of Fred's feelings when the ghost leads him to Fred's house?
It is clear from the conversation that Scrooge overhears thanks to the Ghost of Christmas Present that Fred, Scrooge's nephew, above all feels a great sense of pity for his uncle. Note the following quote where Fred defends his uncle from his dinner guests and refuses to condemn him:
I am sorry for him; I couldn't be angry with him if I tried. Who suffers by his ill whims? Himself, always. Here, he takes it into his head to dislike us, and he won't come and dine with us.
We can see from this quote the truth of Fred's words. Scrooge, whether he knows it or not, only really wounds himself by his attitude and how he thinks about others and treats them. Fred has the wisdom to see past his uncle's prickly exterior and realize this.
In addition, he openly says that he pities Scrooge and that he means to stubbornly invite him to share their Christmas repast with him every single year, no matter how rude and offensive he may be, as he sees the truth of the situation and recognizes that Scrooge must actually be very lonely.