They can deny entry to a minor, if they determine the minor has abandoned residence in the US. A trip of 3-6 months is not going to cause them to think he has abandoned residence. Even a trip of a year or more can be okay, if they maintain ties to the US. Unless they are taking a trip of 3-6 months and coming back to the US for a few days, then going again, etc., repeatedly for years; that would be a problem.
