"Il est un garçon" is not correct French. Good examples on the use of "c'est" vs "il est" can be found here: http://french.about.com/library/weekly/aa032500.htm
They have changed it as i put it's a boy and got the question correct
1) The right question would be :" De quel sexe est-il ?" (speaking about a baby for instance. Here the "il" is neuter) 2)"It's a boy (or It is a boy)" is the correct translation, because if you ask this question, it's because you know nothing about the sex. So "It" is the unidentified for whitch you receive an answer ("It's a boy", It's a girl", "We don't know"). However, "He's a boy" or "She's a girl" won't be incorrect.