"He usually drinks"
Hu- to mark the habitual is always optional. You can use -na- for habitual things too. Because "usually" is in the English sentence though, yeye anakunywa isn't really right because the meaning of "usually" has just gone missing. (Like in English, he drinks implies that it's habitual too, but he usually drinks is very clear about it and it should be in the Swahili sentence.)
these should be accepted:
(Yeye) anakunywa kwa kawaida.
(Yeye) huwa anakunywa.
Mpiranya, Swahili Grammar and Workbook: "To avoid confusion, hu- is generally used with a pronoun or a noun".
Mama/yeye humwona asubuhi: Mother/She usually sees him in the morning.
Huwa hatuwaoni asubuhi: We usually don't see them in the morning.
Many of the exercises on the habitual used to be without (pro)nouns: this has been corrected now. Hence the incorrect capitalisation in this exercise :)