CBP will have his name on the flight list, is his nationality listed also?
I once asked the following question in another thread (I forgot the answer) "what happens if you lose your pp on the plane?" [I've seen people leave pp in the seat pocket, I've found drivers licenses--so losing a pp could happen.] My recollection was, that since citizens must be allowed re-entry to the USA, after verifying citizenship, he will be admitted.