As I understand it, if you move away from the US, his Green Card becomes automatically invalid for re-entry after 1 year, and unfortunately, since he didn't apply for US citizenship before he left, it will likely mean if you want to move back again in the future, he will need to start over with a new immigrant visa again.
What I think that link saying is:
If he knew that he was going to be gone for more than 1 year, he should have applied for a re-entry permit before leaving the US, which would allow him to move back to the US again without needing to start over with a new spousal visa. BUT, the re-entry permit is only valid for 2 years from issue, so if he's gone longer than that it will no longer be valid anyway.
So, from what I understand from your post, his options would be/would have been:
1. If he plans to return to live in the US permanently within 1 year of leaving:
His green card will remain valid for 1 year, so he can move back with no issues
2. If he will be gone at least 1 year, but not more than 2 years:
Apply for a re-entry permit before leaving the US, so that if he's gone for more than 1 year and less than 2 years, he can move back with no issues
3. If he didn't apply for a re-entry permit before leaving the US and/or if he will be gone more than 2 years
Without having US citizenship, he will likely only be able to return to the US if he applies for a returning resident visa (by going through the immigrant visa application and medical exam at the US Embassy in London)
From a quick Google search, I found this document (
https://www.uscis.gov/sites/default/files/USCIS/Resources/B5en.pdf):
A lawful permanent resident (LPR) normally may travel outside the United States and return; however, there are some limitations.
A reentry permit can help prevent two types of problems:
• Your Permanent Resident Card becomes technically invalid for reentry into the United States if you are absent from the United States for 1 year or more.
• Your U.S. permanent residence may be considered as abandoned for absences shorter than 1 year if you take up residence in another country.
A reentry permit establishes that you did not intend to abandon status, and it allows you to apply for admission to the United States after traveling abroad for up to 2 years without having to obtain a returning resident visa. Reentry permits are normally valid for 2 years from the date of issuance.
What will happen if I do not apply for a reentry permit before I travel outside of the United States?
If you are an LPR planning to travel outside of the United States for 1 year or more, it is important that you apply for a reentry permit before you depart the United States. If you stay outside of the United States for 1 year or more and did not apply for a reentry permit before you left, you may be considered to have abandoned your permanent resident status. If this happens, you may be referred to appear before an immigration judge to decide whether or not you have abandoned your status. If you are in this situation, contact the U.S. consulate about a returning resident visa.