Anyone that is a Lawful Permanent Resident (LPR) and is outside of the U.S. for 180 days or more within any 12 months (not necessarily a calendar year) creates a rebuttable presumption of abandonment of residency. Rebutting the presumption is possible by submitting evidence to the contrary, such as filing U.S. taxes, maintaining a home in the U.S. and paying that mortgage or rent, maintaining a U.S. driver's license, U.S. bank accounts with significant movement, etc.
Someone that has been outside of the U.S. for more than one year without first having an approved re-entry permit has abandoned their residency, and only in very few exceptions (such as serious illness) can they get it back.
Here is an official link:
https://www.uscis.gov/green-card/after-green-card-granted/maintaining-permanent-residence
That may become a problem in a trip back into the U.S. when applying to renew the residency card or applying for U.S. Citizenship.
So basically, he needs to spend more time in the U.S. than outside. If that will not be possible, at least at the beginning, he can apply for a Re-Entry Permit on form I-131. That will allow him to be outside of the U.S. for up to 2 years. He can use it for multiple trips. If he needs more time, he comes back before the two years end, and then he can apply for one more Re-Entry Permit for two more years. If he still needs more time, he can apply again, but after four years, they get harder and harder to get and often only grant them for one year at a time. At some point, he will need to start living in the U.S. and spending more than 180 days per 12 months inside. Here is a link to the I-131:
https://www.uscis.gov/i-131
If his biological parent becomes a U.S. Citizen BEFORE he turns 18 and he is living in their legal and physical custody also before turning 18 and also has a green card, then he becomes a U.S. Citizen automatically:
https://www.uscis.gov/policy-manual/volume-12-part-h-chapter-4
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