Take someone with you or follow him and get his tag number or address. I would be ticked off too, in fact it does make me mad. What is wrong with people???
What are your options?
The police who will do nothing about some tee shirts stolen.
You can ask for the back but there is no telling what you may get.
You could "get him back" by sashing his tires or something but that would make you as bad as him.
You could hire a couple of bikers (or Guido) to get your shirts once you figure out where he lives. That might be the best option unless of course they hurt the guy and say you hired them to do it.
I don't know if there is a good option. Perhaps you should take him a couple of more tee shirts, give them to him, and tell him that he has them all now. Tell him he obviously needs them more than you do. Then go get yurself some new tees.
J