Well, firstly, "widzieć" (to see) takes Accusative. In Accusative, for masculine nouns, you have to take into consideration whether the noun is animate or inanimate. "chłopiec" (a boy) is, logically, animate. The Accusative form of animate masculine noun is identical to the Genitive (while for inanimate, identical to Nominative). Thus: "chłopca". And obviously the pronoun has to match. "That" = "Tamten" (for masculine nouns), so the whole phrase, taking Accusative, is "Widzę tamtego chłopca".