Yes, he needs to get a new one.
The thing is that he is going to Mexico and no passport is required for a US citizen to enter Mexico. The passport rules are set by the country(ies) that you are visiting. If he was going to, say, various ports in Europe, he would not get vary far.
Also, it is the State Department that sets the rules for passports not Homeland Security or Carnival.
SS Norway (12 times) '85 - '97; Radiance OS 06/01; Island Princess 11/04, Grand Princess 11/04/05, Golden Princess 7/21/06, Golden Princess 2/24/07, Explorer of the Seas 12/23/07, Island Princess 11/15/08