I would have insisted on a X-ray on the spot....

Not meaning to scear you here but this is why I would want a X Ray. I had a friend who had a wonderful dog, this dog had saved her life several times. Bonnie started limping, after months passed, she started chewing on her paw on that leg so she took her to the vet. X-rayed, said arthritis and gave her meds for that. Bonnie continued to get worse even after changing her meds a few times so she decided to take Bonnie to another vet. I was there when she got the call from the vet and we both almost passed out with the diagnoses.. CANCER! Vet #1 X-rayed from the elbo down... Vet #2 X-rayed the whole leg.... So if you do the X-ray, have him do the WHOLE leg.

My guess with Scout it IS arthritis or a sprained muscle...