
I'd go back & (preferably) see another vet. I took a limping dog once & was given antibiotics ( infected bite, well puppy play bite really) When the AB were finished he was still limping so I took him back & the (different) vet wiped the floor with me "How long has this dog been like this?" he had a broken metacarpal (meta tarsel?, small bone anyway) & ended up in plaster. I know you say your dog was xrayed, but did they xray his foot?
Anyway, if your worried, take him back. If it is ligaments, I think it can take a while, but would have thought you'd see a little improvement by now.