Seriously? It is 100% correct. It is the correct translation for this sentence. (Otherwise it would be the boy drinks milk).
This sentence can be used when there are a boy and a girl at a table with different types of drinks. The boy drinks the milk. Or actually ..just different drinks would work too. Wat drinkt hij, de cola? Nee hij drinkt de melk. Niet, hij drinkt melk.
Im dutch too btw these exercises aren't about "translate it to the most common sentence it looks like". Je moet vertalen wat er staat, niet er iets anders van maken. Ook al is de zin ongebruikelijk. In beide talen.
Dutch nouns have gender. "De" is used with the common gender (there used to be and technically still are separate masculine and feminine genders, but for practical purpose, they're collapsed into a single common gender) and "het" is used with the neuter gender. There are ways of guessing the gender which you'll find in any Dutch grammar book, but some of the time you just have to learn what a noun's gender is.