Putting hand through collar of shirt

My son will be 2 in a little less than two months. For the past couple weeks, he has been putting his hands through the collar of his shirts or sleepers and either is messing with his nipple or his armpit. I try to get him to not do it, as it’s stretching out his shirts. But I’ve been unsuccessful. Anyone know what I can do to try and prevent him from doing that? And why he’s doing it? I’ve read it could be a self soothing thing, but he doesn’t seem stressed or anything. He’s generally a happy baby.

**I noticed he DOES do it more often when he’s ready for a nap. He also pokes at his belly button just as much, and today I noticed it was blistered from him messing with it. :( I try giving him a toy, and it helps a little. He won’t take a pacifier. I’ve tried a few times and he doesn’t seem interested. Now I’m worried about his belly button. I’ll try to find a more comforting toy maybe a lovey blanket? Would that work or would he be too old to have an interest in those?