This sentence is trying to say that he is feeling more than just the average sympathy. As in, if a human is only capable of this much sympathy, he felt more. Does that help?

Here /human sympathy/ means /sympathy for another human being/, so a clumsy but clearer rewriting of the beginning of the sentence would be: - I felt more than ordinary sympathy for another human being ... The term /sympathy/ has its origin (from Greek) in the meaning /to share feelings/, and usually /human/ is implied, but we can also /share feelings/ with a dog that is suffering.

I strongly believe that you picked the wrong explanation.