Given that the reveal of Joe’s death was only addressed in the tie-in novel that many Arrow viewers may not have read, there’s a chance that the folks behind the scenes decided to just skip over this particular detail and establish Joe as alive to give Slade a reason to join Oliver.
The Arrow tie-in novel Vengeance revealed that young Joe Wilson had been killed by his father when Slade was in a Mirakuru rage.
Unless Oliver is a huge jerk and Joe’s “present whereabouts” are a grave somewhere, Slade’s son is evidently very much alive.
It wouldn’t be the first time that continuity was sacrificed for plot on Arrow.
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Arrow 5x23 Sneak Peek "Lian Yu" (HD) Season 5 Episode 23 Sneak Peek Season Finale