I recently returned to the US from another rout. After he gets off the plane there will be an interview where they can ask a lot of questions. For me, they were, 'Where have you been?', 'What were you doing there?, 'Did you bring back any plants or animals?'.
I know they will ask him what is in his bags -- food items like plants and fish for sure. They will likely ask him why he is entering the U.S. and how long he is going to stay. Good luck.
