if a green card holder is contemplating a departure from the US for more than 6 months, obtaining a reentry permit is a good idea. Depending on this person's age, he should also investigate whether he has acquired US Citizenship already.
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If I were you, I would apply for a reentry permit. This document enables you to return after an extended stay, usually around a year. With my local LA clients, I've had very good results obtaining them. Consider hiring a local attorney to help you, but get a free consultation first. I always say any attorney who won't give you a free consultation doesn't deserve your business. If they don't have time for that, how do they have time for your case?
The father should see an immigration lawyer to determine if the child is not already a US citizen. If that is the case, there are no immigration restrictions on his ability to attend school full-time in Nepal.
You are right to be a little concerned. A citizen is not required to maintain any residency requirements, and is free to travel for as long or as frequently as they like. However, that is not so for a legal permanent resident (LPR). A LPR must maintain his resident status. If he wishes to leave for more than 6 months, it is advisable that he apply for a reentry permit and to have an attorney help him with this so he is sure to maintain status. If he is eligible for citizenship, it would be the best idea to apply for that ASAP, and maybe hold off before he goes since leaving for more than 6 months will disrupt his physical presence required by the USCIS to adjust to Citizen and prolong that process.
You may be found to have abandoned your permanent resident status if you:
•Move to another country intending to live there permanently
•Remain outside of the United States for more than 1 year without obtaining a reentry permit or returning resident visa. However, in determining whether your status has been abandoned, any length of absence from the United States may be considered, even if less than 1 year
•Remain outside of the United States for more than 2 years after issuance of a reentry permit without obtaining a returning resident visa. However, in determining whether your status has been abandoned any length of absence from the United States may be considered, even if less than 1 year
•Fail to file income tax returns while living outside of the United States for any period
•Declare yourself a “nonimmigrant” on your tax returns
Often people that obtained their immigration status years ago (like his father may have) tell people that gaining or maintaining a benefit "is no big deal" or "it's easy", but that is not so anymore. Immigration has become very tricky and the USCIS has gotten a hard nose in recent years. So, it is a good idea to at minimum consult an attorney personally to get the best answer for the situation.
Best of luck!
This is not legal advice and is only general information. I am not your attorney. No attorney-client relationship is formed by this communication.