Is he over weight? it sounds as if there could be some issue causing this , he says his stomach hurts it may not be from tight pants ,maybe a check up at the Doctors would be good.I think perhaps his mom should buy his right size why has he got tight pants , if he has grown out of them maybge his Mom needs to buy some that fit, he wont go to school in his underwear .
This sounds like mild sensory issues.
My middle son - who has now outgrown it - used to insist on wearing "square pants" - those waffle weave long underwear in a size too small, and a pair of nylon bicycle shorts over that, and then jeans. And a turtleneck shirt, too small, and over that button down denim shirt, tucked in, with a belt. Cowboy boots, one size too small, when he was 4.
Being wrapped up tightly in clothes made him feel better - you could just see it in his face.
I wonder if your nephew would wear those cool underarmour workout underwear - it would give him the "tight" feeling without being actually uncomfortable.
This too shall pass.