I went through something like this with my sweet little Jet, who passed last February. He had multiple old age issues plus a chronic ear infection and E.C. I found his appetite in timothy hays pretty much went away. He was also blind (cataracts) but could navigate quite well. However, his robust-young-bunny-days appetite was gone.
I resorted to alfalfa hay only (there was still Timothy around but he didn’t eat it) along with veggies and diced papaya.
That was still not enough. I had to go to assisted (syringe) feeding two… then three and finally four times a day. Around 30-40cc each time. I made a blenderized mixture of Critical Care and Oxbow’s Alfalfa pellets. (There is a definite skill in getting pellets to blenderize). It did work until about the last month of his life when his appetite slowly wasted away. Nevertheless, I felt it was worth it for the time offered. The key is you will have to feed him; offering it won’t do any good. When they get older, their appetites wane. Normally I prefer a skinnier bunny but not when they are older. It’s a sad reality that you will probably be fighting weight loss with him from here on out.