。
Next year he'll have been living in Paris for ten years.
Literal
Next-year [at-で] he [topic-は] 10-year Paris [in-に] is-living [ことになる] (it-will-be-the-case).
~ことになる anchors cumulative duration to a future point. The choice of 住んでいる (ongoing) vs the past 住んだ affects nuance: 住んでいる emphasizes that he's still living in Paris and will continue to be, while a past form would foreground the completed span. Pairs nicely with how English uses 'will have been living' rather than 'will have lived' for emphasis on continued state.