I answered this question with "I know him" but this was marked incorrect, with the correct answer given as "I know".
This was in lesson 1 of Clitics so I presumed Lo was the pronoun for him (or it). Was I wrong or should I report this?
The verb "sapere" can't be used to say that you know a person. "I know him" would be "lo conosco". "Lo so" can be translated as "I know" or "I know it", but "lo" is anyway referred to a thing, to an information.
Pay attention to the verb "to know", because in Italian it has two different translations depending on what/who is known. "Io lo conosco" is roughly "I'm acquainted/familiar with him", while "Io lo so" is used only for things or concepts. This link may help you understand it better: http://forum.wordreference.com/showthread.php?t=247134&langid=14