I admit I am a big softie, but I would see the shirts as a "security blanket" of sorts. For some inexplicable reason he has developed an attachment (though illogical to you) to these shirts. Maybe he has a very fond memory from each one. God knows our kid's have minds like steel traps - he can probably remember everything that happened on any given day, just by looking at those shirts. Whatever you do, just remember, and you do -seem to "get it" As silly as it seems, his attachment is real, and I do not think it should be forced on him to abandon his "blankie. He has a real need for these shirts, I think the wearing his chosen shirts under new ones is a great idea - if he will do it. What about buying the exact shirt, one size up, and washing it a bunch of times so it resembles the old one more. Is there a snag on the old one? A stain even - put one there. Have you seen the commercial where the mom takes a brand new teddy bear, washes the car with it, runs it over, and pulls off an eye - then gives it to her dd who is so glad mommy found teddy? try something like that, w/o him noticing. While he is at school, simply get it all ready, put it where HIS shirt should be and try that. As nonchalantly as possible. If he throws a fit, he is just not ready. He may look a bit messy - but his happiness is more important than what people think. He will get over it eventually (hopefully before he is TOO big!)
In the future, once he gets i=over the shirt thiing - I would try to mix up his clothing routine often. My ds used to get so completely freaked out when the seasons changed and we went from short sleeves to long sleeves shorts to pants and vice versa. So I made a point of having hime wear the off season choice at least once a week, so he would remain used to the feel of the different clothing. It really helped him, now his only thing is NO TAGS! Can't blame him