"I know the boy."
Think it like this; You can know a piece of information, but you can't really know a person, or a street, or anything that actually EXISTS. That sort of knowing is to italians about being familiar with them. You can KNOW a piece of information, but you can only be FAMILIAR about the existence of physical things or people.
Unfortunately there is not a precise rule! It is difficult to explain also for me. "sapere" is general, "conoscere" can be instead used for words/nouns that represent people. But not always. Usually, "sapere" is escorted by "chi (chi)", "cosa (what)", "quando (when)", "che (that)"; Example:
Io so chi... Io so che.... Io so quando.... etc.