Jonah should be there from the start; you don't need to wait or do anything to trigger his appearance. He is not in Vacca's haunted cave, if that is where you were looking.
From the beach (IIRC) teleporter shrine (near where you start on the island, on the opposite side of the hermit's cave), look west (out to sea). The windy peninsula with the grave should be off to the right, while Jonah is at the very end of the peninsula to the left (by a fishing pole).
Sentinel Island map (from D2:ED, but the geography is the same in DKS)
Once you bring Jonah the necklace that Catherine gives you he will disappear. If he was never at that spot, email support@larian.com.
Alternately, you can just skip the 2 side quests he is required for.
If you are stuck on what to do next, here is a hint that may or may not be relevant;
You got a book of magic (Arcane Notebook) that can control wind when you rescued the guy on the top of the pillar (near the trainer Kenneth). There is a windy spot near where you start on the island with a tombstone and three candles...
Speaking of the windy peninsula, have you done any swimming?
Jump into the water to either side of that peninsula, and you can swim to the entrance of a hidden cave.
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