I was attracted to him because of his looks.

his 是形容词性物主代词，表示"他的"，用来修饰名词。 looks 是名词 look 的复数形式，在这里指"外貌、长相"，特指一个人整体的外表特征。 his looks 表示"他的外貌"，强调这个人整体的外表特征。