Well, I just took him down to the main (my parent’s) house to clean him up (pipes froze last night, and they just got fixed, so water is incoming at the moment.) He doesn’t seem to still be doing it. When I wiped up the fluid it was pure brown.
Thankfully, as I found when carrying him and trying to clean him, he’s acting completely normal. He bobbed his head curiously when we walked to the house, he kicked a bit when I tried to check him on my own (ended up having one of them hold while I cleaned him.) Now he’s sat in his cage grooming himself.
Other recent information: Last night he was thumping up a storm, but I figured that was completely related to the fact that he had a bunny bonding session, and he could still see the bunny, and still wanted to play with her. I covered up his cage and went to bed.
He has had celery recently, which is a new addition to his diet. (I’m trying to find a treat both rabbits like enough to enjoy during bonding.) It wasn’t a whole stalk, just a third of it (the end with the parsley-looking bit.)
Edit: He tried to jump onto his food area, and fell off. I then offered him a treat (banana chip, his favourite thing in the world), and he took it, ate it, and then asked me for another, so he’s acting about 80% normal.