This is so funny! I am giggling because when I was a little girl, I was obsessesd with shirt collars! Yes, my parents thought it was weird, and if they were concerned they did not show it. But I am a child of the '70's, and so you recall everyone had these fabulous wide, big collars as part of the fashion trend at the time. And lots of people wore polyester, so the collars were nice and smooth. I would find a collar, rub the pointy tip with my finger, and suck my thumb! My dad's old shirts from the '70's - he still had a few and you can see how the edges are all browned from my rubbing. I don't know what it was, but I would find the smoothness of those polyester collar tips soooo soothing and relaxing to touch. Sometimes it would even make me drift off to sleep (maybe that's why my parents never stopped me from doing it, it worked their advantage!). When I stopped sucking my thumb at 5 years old, I stopped rubbing collars. That was that! Although even now when I see I nice tip on a men's shirt I will rub it (only a little! and only occasionally, really!)
Now, my son....when he was about three weeks old, he started making these abnormal twisting movements with his mouth, randomly. Of course, I worried (is it a seizure? i wondered). My mother was perplexed but she insisted that it was probably normal and self soothing behavior. She was right. Now, 3 years later, when my son is sleepy or cuddling with his blankie, he will still make those twisting mouth movements (which I later realized what he is actually doing is sticking the tip of his tongue out, catching it between his teeth, and "sucking" on his tongue like a pacifier). It's so weird, but so cute! It makes him unique, I think.
That collar story is bringing back so many fond memories and as I am typing this I am totally cracking up between your son's story and my own! LOLOLOL! Anyway, I figure, since I stopped, one day your son will too. Enjoy it for now! Make sure you videotape it so you can have a good chuckle years from now!