Thanks, guys! The pig is currently in a 2x3 C&C. My suspicion is that he doesn't get enough hay, because my niece insists on giving him his hay in a small little ceramic bowl instead of a hay rack or hammock like most owners on here seem to. Like just a small kid-sized handful of hay. I told her he needs a constant supply of hay, and she keeps assuring me he has one, but I find it hard to believe a little handful of hay will suffice while she's at school and her parents are at work.
My Dad told me to make them a hay rack like the one I use (I know my pigs whack hay like there's no tomorrow.) Here's a photo:
But I'm not sure my niece's family will use it b/c the kid got very upset when I put a large (grownup-size) handful of hay in the cage instead of a neat little kid-sized one in the ceramic bowl.
That's another problem. My niece seems to have her own little system for taking care of the pig (the cage is in her bedroom) and gets upset if it seems like I'm criticizing it, no matter how tactful I try to be. And her parents just get mad at me for upsetting the kid even though that wasn't my intention.
I will try to follow your advice, Lynx, although it might be difficult since I don't see them all that much, and as I said above, i'm dealing with a family who is very set in their pig care ways, and who don't seem to mind the soft poos and the smell since the pig seems otherwise healthy. But if I'm going to be called over every time they need to clip his nails, I'm going to tell them they gotta learn to clip his nails themselves (I linked the GL page), because I am not here for scraping poop off with my fingernails and the smell legit makes me want to boot. Maybe they are used to it so they don't mind as much, but for me, it's always an unpleasant shock. It makes me feel bad to say that, but you know, I already got two piggies of my own to take care of. I'm glad to help, but they gotta learn sometime.
My brother, at least, seems more amenable to taking my advice and was asking about a more open hidey than the pigloo, like the one I use (it's just a desk organizer shelf thing I got at an office supply store, and then pinned a scrap of fleece to as a "curtain"). My niece is a good kid, but is still a kid, and I don't feel very comfortable with her being the pig's primary caretaker. It bothers me that she was the only one observing carefully during my nail clipping adventure. I should've insisted my brother watch since he's going to be doing this next time. Sigh.
Thanks again for the responses!