Did you asked him if he knew about bitcoin? It's possible that a relative bitcoiner just gave it to him as a gift or maybe a giveaway shirt passed down to him. I know many elderly people who doesn't care about the print on what they wear as long as it's comfortable.
I saw early today an old man (about 75 years old) wearing Bitcoin shirt.
Clicked on the thread title hoping to see a photo of Jet Cash. Sadly disappointed.
Come on! hehehe.