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How old was your son when you became a U.S. citizen? Your son was very lucky that he did not have any problem with CBP each time he re-entered the U.S.
The rule is for the past 5 years, your son cannot be absent from the U.S. for a total of 30 months AND each absence cannot generally be more than 6 months. If over 1 year, he is automatically ineligible for naturalization until such time that he is eligible based on the rule mentioned above (4 years and 1 day after return from 1 year absence).