[Ellie May]

With that much weight loss, bloodwork should be done.

Has the vet diagnosed the leg problem (told you what is causing it)? It could be so many things, but common arthritis ay be possible. When an arthritic dog walks on flat ground they shift their body weight to the front (assuming arthritis is toward the back). This helps them move. When they are going up, they can't do this making it more difficult. Have pain meds been tried? Pain could also stop a pup from eating.