Might lose my kitten

My newer kitten has been tiny since birth, he was the smallest in the litter. I've been worried about his size and eating, so I took him to the vet. He has no muscle mass and no fat, he's almost 6 months old and only weighs 1kg, and very little appetite no matter what food I offer him. The vet basically said that we don't know what's wrong with him until some tests, but that I should emotionally prepare for him to be put down. Sorry for the vent but I had to let this out somewhere. I also know this isn't technically a pet loss, but the vet made me feel like I've already lost him.