"You know a boy."
"Ty" is "you" when you address one person.
"Wy" is "you" when you address more than one person.
There are also formal versions which you will encounter later, which are quite needed because it would be rude to address a stranger "ty", especially if he or she is older. "Ty" is for your friends, for children, for people you feel really comfortable with.
It doesn't really state whether the statement is based on more than one person.
Yes. That is exactly why "(Ty) znasz chłopca" and "(Wy) znacie chłopca" are accepted :)
There are a lot of sentences which are more ambigous in English than in Polish and you will simply have to choose the version you're going to write.
English "to know" has two translations: "znać" and "wiedzieć".
"znać" is more as "to be familiar with", "wiedzieć" is "to have knowledge".
Therefore I would say that znać is "to know X", and "wiedzieć" is "to know, that X" or "to know about X".
So no, "wiesz" doesn't suit this sentence.
The verb "znać" (to know) takes Accusative.
Basically, the direct object is usually in Accusative.