To get past the man to enter the Cave of Origin you need to reach a certain point in the story. You will not miss the Cave of Origin as the story directly tells you go there once you've reached that point.
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After groudon and kyorgre fight you can go inside the cave
u have to awake raquaza
Groudon can be found at: Cave of Origin, Terra cave.
Find the submarine Team Magma stole and defeat themand awaken Groudon
beat the freakin elite 4 get the nat dex