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There is nothing that you have to do. If he wants to come to USA, he has to file for a B2 visitor visa. He can do this at the US Consulate or Embassy nearest to his permanent residency. This is all on HIM.
The only thing that you may do (optional) is to send a letter to the US consulate/embassy once his B2 application is underway, which speaks for his good character. See sample HERE.
This is optional, however. Aside from that, he has to initiate and see through the whole process.
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