For those wondering about the use of kluk, chlapec, and hoch, I would say "kluk" is the most widely used and should be your go-to word when saying boy.
Chlapec is rarely used in everyday conversation, would probably only hear it if an older person is speaking very formally or more frequently you would see it in a news article or on the news where they tend to speak in more formal/proper Czech. (So more of a word for you to recognize than use)
Hoch, I've personally never heard said as a word for boy, but only specifically in the phrase "Hele hochu,..." as in "Look here boy/Hey boy", but in that use it doesn't necessarily mean boy, as it's used that way by older men to any man significantly younger than them. (My great uncle uses it when speaking to me, I'm almost 30)
I am not Czech but I heard the word "hoch" about two months ago when a young guy called his friends and told them to tidy something up so it was like "Hoší, uklidněte tady". And only thanks to DouLingo I understood him, because I had not been aware of its existance in Czech.
"Chlopak" is not a Czech word. Sounds Polish. There is a Czech word similar to 'chlopak' and it is 'chlapík' which means 'a guy' ' a chap' and I am not entirely sure if it is considered street Czech or proper. In any case it is an adult person. You might have meant "chlapec". There is no difference between hoch and chlapec
Is there any reason to have so many different words for the same thing so early in the course? Kluk, chlapec, and hoch are all very different words and serve the same purpose, as you say. It seems very confusing as part of these tasks, when the focus should be more on the grammar...