We Never Believe Time Travelers

A man in Casper, Wyoming was arrested for public intoxication. He claimed to be from the future. From the year 2048. He came back to warn us about a coming alien invasion. And apparently a requirement for time travel was that his body be filled with alcohol. The invasion is coming in 2018, which was the year he meant to travel back to. If he really were from the future, what would it take for us to believe him?