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".
"chłopcem" is Instrumental. Like in "I am a boy".
"widzieć" (to see) takes Accusative. "Widzę chłopca".
I wrote the exact translation and it was considered wrong. Did not have the usual option that my my answer should have been accepted when I tried to report the problem. I know there are computer glitches. Just reporting this one.