I suggest he travels back to the United States as soon as possible and applies for a LPR card renewal at once as you cannot do if from overseas and his admission could be problematic.
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How long has he been out of the US? This is going to be the first question that the CBP officer will ask when he attempts to re-enter with an expired greencard.
I suggest he make arrangements to come in before the card expires. Or contact the US Consulate about a SR-1 returning resident visa.
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To avoid tremendous complications which may culminate in Nephew losing his green card status altogether, best to come back to the US prior to that green card expiring, irregardless of whether have re entry permit or not. Reentry permit does not guarantee one's admission back, and certainly does not protect from CBP's assessment at the airport that one has in effect "abandoned" his US permanent residence.
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