Could this be coccidia? I was just wondering if that could be what the mass is - a swollen liver or something like that? Here's a link to the symptoms and other info:
https://rabbit.org/intermittent-soft-cecotropes-in-rabbits/
I would definitely try to find someone in your area that has experience with rabbits. If you do a google search for your city, country, and the words veterinarian rabbit, does it not give you anyone there who might be able to help you more? I tried this with the city listed on one of the reports and it listed a few with one who said he had over 18 years as vet and could treat rabbits. It's so hard to take what you are seeing and the reports and help you diagnose or make choices as to what to do next.
Our rabbit had bladder sludge and a UTI. We had to finish the course of antibiotics, but we also had to get him some sub q fluids which seemed to help. Our vet also said they needed to move more, to help prevent the sludge in the future. Our rabbit was peeing outside of his litter box six months later, which is what he did when he had a UTI the first time, so we gave him a small dose of metoclopromide and some Metacam (with our vet's approval), and he improved without needing antibiotics. But I wouldn't give metoclopromide to a rabbit who is having liquid cecotropes.....because it could make it worse. Are all his poops liquid or only the cecotropes?
I don't know I would give any cranberries, as they might make things worse. Are you giving any greens? Like a leafy romaine lettuce? I would give those if he will eat them. That will provide some water for him.
Pain meds could help him feel good enough to drink and urinate more. I'm so sorry you are going through this.