And if the vet tells you it's his shoulder - call a different one.
Unless there is swelling or another outward sign that would be causing lameness in that area (ie a cut or kick), it is very unlikely that the lameness is originating in the shoulder.
If the vet can't make it out for a while, cold hose the leg and restrict his movement (stall rest or in a small pen) until he can make it out to make a diagnosis.
Good luck!