My boyfriend's green card is expired and has been for 4 years. Is it too late to get it renewed? Has not left US anytime during the 4 years

Permanent residence is permanent, and it doesn't matter if he has traveled (unless outside the US for more than 6 months).

File form I-90 to get a replacement green card. He will be scheduled for biometrics (10 print and digital picture) and then after the FBI check is completed, will be sent a new card.

Please remind him of his ongoing obligation to file form AR-11 if he changes his address. He should consider filing for citizenship if he qualifies.

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