Thank you so much for the wonderful response!
I was half joking to myself that he'd wind up pooping right after I went through the trouble of making this post and worrying about him for weeks on end, and it turns out that's exactly what happened. I didn't know that underfed snakes sometimes go less frequently in order to get the most nutrients out of their food and to retain water, but it makes sense. I took some pictures of him today that I'll try to figure out how to post after I submit this. I don't think he's horribly underweight but he's definitely skinnier than other ball pythons I've held. Right now he's 922 grams and 43 inches long.
Getting him on rats is definitely the goal if he'll eat them, so I'll make sure to offer him one while I still have some of the mice left in case he doesn't take it. I weighed a few of the mice and they're 19-20 grams, so I guess they'd fall under the category of adult according to the chart but they're still way too small for him. I'll keep feeding him on a weekly basis and slowly increase the size of his mice/rats.
He has mulch as substrate but I put a Tupperware lid down when I feed him and watch to make sure he doesn't accidentally eat any so fortunately I don't think that's an issue. On a side note, what substrate would you recommend? I want to get him something that'll hold the humidity better but there's a lot of debate online about what's best. The mulch he has now dries out quickly and I have to spray him down often. Heating is something that I've been working on fixing since his ambient temperature is definitely too low, but it's hard to find a heat lamp that has a cord long enough to reach from the top of his enclosure to the power strip that I have on the floor. I live in a very small apartment and the only place I had room to put his enclosure was on the top of a tall dresser, and I'm hesitant to run something like a heat lamp off on an extension cord especially since it would have to plug into the power strip which already has a lot of things hooked up to it. When I move to a bigger place in the (hopefully) not too distant future I'll be able to get him a more ideal setup.
I'll keep an eye out for any lumps near his cloaca in the future, but if there aren't any and it's been a month or two I'll just assume that he's still trying to get as much nutrients out of his food that he can. Hopefully once's he's eating enough it won't be an issue at all.
Thank you again for the great information!