Thanks Eole. I took an initial girth measurement when we got the horse so I have something to compare to. I will be re-measuring every week. So far, he seems ok, but it's only been a couple of weeks. He does get fed a very small amount of pellets twice a day in his stall. Also, once the winter weather comes, they come in at night, so if necessary, that would be a good time to give him a few extra flakes since he wouldn't have to share. It's not ideal - I'd prefer a slow-feeder of some sort so he's not bored - but we may not be there more than a couple more months.
That said, I'm also unhappy about the amount of nipping my horse is taking from the other horse (another boarder has told me she had her horse taken out of that paddock because of this). I get that horses will nip at each other, and maybe I'm just being an overprotective horse owner, but he has 5-6 big marks on his rump. None are very deep (one or two drops of blood on a couple), but I worry about whether the hair will grow back. My daughter would like to show this horse so all these scars are not good.