However, his whereabouts have been unknown for 6 years.

his 是代词 he 的所有格形式，表示"他的"。 whereabouts 是由 where（哪里）和 about（关于）组成的复合名词，表示"下落、行踪"或"所在之处"。 his whereabouts 指"他的下落"或"他的行踪"。