。
I like him all the more for being quiet.
Literal
Quiet [because-だから] all-the-more he [subject-が] is-liked [explanatory-のだ].
無口 = 'quiet, taciturn' (of a person). なおさら ('all the more, especially') intensifies — 'because he's quiet, that makes me like him even more.' An unusual compliment reflecting the Japanese cultural appreciation of reserve.