"Do you see a boy?"

Translation: Widzisz chłopca?

[maxik43]

"Czy widzisz chlopca" means the same as "do you see a boy"

[Jesuis89]

So I can't say "widzisz chłopiec?"

[Sean_Roy]

No, the direct object has to be in the accusative case: chłopca.

[RanRutenberg]

I think that a noun that follows the verb widzieć has to be in the accusative declination