Some games yes, some games no. I don't know which games you mean...
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You will need to unlock the National Dex and defeat the Pokemon League to make him move.
In Pokemon Platinum to get the National Dex you will need to have seen all 210 Pokemon in the Sinnoh Dex then talk to Professor Rowan and then after a while Professor Oak will give you the National Dex!
Not really. You can still get Oak's Letter without it, but to get TO route 224, you need to beat the E4 and have the National dex.
No you need the national dex.
Yes.