He's probably a douche bag, but there is a chance that it's really a shirt mocking the Yankees that you can't see. I have a shirt that has the NY Giants logo, but spelled, "Ginas". A couple of Giants fans didn't realize what it said and thought I was a G-Men fan.
No trust me I've been to enough Yanks/Rays games to know these idiots only pull their Yankees gear out when they are in town otherwise they are rocking Rays gear.
Dawk I've seen with my own eyes after a Rays win a Redsox fan take off a Sox jersey and had a Rays t-shirt underneath it.
Mushman is right - I went to a Rays/Yank-mees game at the Trop a few years ago and was chatting up some friendly Yankee fans - they told me that when the Yankees are in town they root for them, otherwise they root for the Rays - unbelievable!!!