Unless he's going swimming, my 3 1/2 year old son always wears a long sleeve shirt and pants (not jeans and not button down shirts). I'm not expecting a 3 1/2 year old for logic and but I don't think this is merely psychosis, just stubbornness and a desire to assert his own will.
I think he needs to wear less clothes, especially as hot as the summer has been and his clothes are just plain old wearing out. We've got 3x more t-shirts than long sleeve shirts and they've never, ever been worn.
What's a good way to coax a little man out of his style, just for the sake of prudence.