What kind of a dog is this?
How long has he been being treated for the pneumonia and does he seem better from that?
Labrador retrievers can have a health issue where they become weak.
Hereditary Labrador Retriever Myopathy is one issue which you can read about here
http://www.labbies.com/hmlr.htm
Dystrophin Deficient Muscular Dystrophy is another issue which you can read about here
http://www.gopetsamerica.com/dog-health/canine_x_linked_muscular_dystrophy.aspx
But it could be the pup is weak in the rear due to being ill and not moving around much, It might also be he has bad hips or a knee problem.
He could have a tick carried disease or have had an accident before you got him where there was some spinal damage.
When your vet is available I'd plan another vet visit to address the leg problem.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics