I’m sorry your bun is having problems. Since you took him to a rabbit vet and had test done, this could be about ageing. Arthritis (joint pain and inflammation du to age) can be a factor. Pain can make a bun less inclined to move around, and that obviously affects the bun’s capacity for maintaining skeletal muscle. It’s seen in old horses and v old humans too. Hopefully the pain med you got (Im guessing meloxicam) will help with that.
Junior pellets often contain alfalfa, and as you are saying, alfalfa has a lot of calcium. High levels of calcium is not good for adult rabbits. The uptake of calcium is not as tightly regulated in rabbits as in most other mammals, meaning rabbits practically take up all dietary calcium. Excess calcium can lead to calification of blood vessels and inner organs. Your vet might have tested his blood calcium levels so they knew alfalfa wouldnt be a good option for him. (I had a bun that had high blood calcium, I was told to cut out kale and other dark green vegs from his diet).
The higher protein content in junior pellets is probably good for elderbuns though, just like it is for elderly people, since the body pretty much consists of protein (and water obv). Rabbits however get a lot of the amino acids they need from the overturn of the microorganisms in their gut.
Daily Critical Care (there are other recovery foods that a bun might prefer) plus rolled oats, sunflower seeds and carrot (obviously not in any large quantities) helped me keep weight on my elderbun. I also gave him rabbit probiotics.