I think his fear is unfounded. Sure, there are incidents there, but far less than in the USA, and almost 100% involving people who are involved in the drug trade. For tourists, who stick to the tourist areas, Mexico may be one of the safer destinations.
Now, that said, appeal to his logical side. For instance, in Cabo, it is not on any drug route, and is very safe. The stores are familiar- like Costco, WalMart, Target. Cabo Wabo- Ok, maybe nobody's immune to a hangover, but who can turn down a good-ol' American Margarita/sports bar? Cabo is very safe, and very un-Mexican. Hopefully a little research will convince him. Y'know there are self-contained resorts there. Take the resort's shuttle from/to the airport, and never leave the beautifully manicured grounds. He'd hardly know he wasn't in San Diego or Hawaii, or Key West.
Good Luck!
Jim