I need to find supplements & ways to increase muscle mass for a decade-old male cat who has needs to be careful of his kidneys. Amino acids, l carnitine, lysine, herbal or natural steroids - what would be most helpful & less risky? Thanks.
Have you asked your vet?
Not yet. Just getting in always seems to cost a bundle.
Why do you need to build muscle in a cat? Have you tried increasing his activity level? How about some new interactive toys?
Indeed, OP, does your cat even lift? 
OK, that’s funny L
Seriously, he got sick and suddenly lost a lot of muscle mass. Especially in his shoulder area. His skin hangs there. Walking him on a leash doesn’t build up that muscle. Getting him to jump repeatedly doesn’t build that muscle. Nothing seems to help to restore his muscle there.