Has his his water intake increased?
Rumball is thin these days also.
I try give rolled oats and pepitas and sunflower seeds to help. Vet said the omega oils in the seeds would be beneficial for him too. She’d also said older buns (and animals in general) just don’t derive the nutrition from food as easily as when they are younger.
I also have hay pellets (98% hay) that I give sometimes. Less work for them breaking it down in the body.
Interestingly, it was when I started Rumball on daily meloxicam that I saw a slight gain. I’d also treated an ear infection and I do wonder if his immune system was taking all the energy her gets from food. So supporting the immune system for older rabbits would likely help, in my view.
I also discovered he enjoys body warmth when I have him on my lap. So keeping warm for these older guys may be a tad harder. Plus, heat is soothing for arthritis. I bought one of those passive thermal mats (reflective stuff inside) and Ailis and Rumball spend 95% of there time on it. Even now when our weather is warming up here.