Joe Swanson originally had a son named Kevin, who was eventually written out of the series. His disappearance was later explained in "Stew-Roids" (#7.13) as that he had died in Iraq. Kevin later appears at the Griffins door in "Thanksgiving" where it's revealed he may have deserted from military, As at the very end of the show a guy claiming to be the REAL Kevin Swanson appears at the door to which Peter replies "I don't think there's time for this guys".
The trivia item below may give away important plot points.
The man at the end who claims to be the REAL Kevin Swanson might be a reference to the infamous 'Simpsons' episode, "The Principal and the Pauper" which reveals that Principal Skinner is actually an impostor who took the real Skinner's place when he was missing in action during the Vietnam War.