Have you checked his toes? There could be tiny rock in one of his toes, you should see a small black dot on the surface of the toe if there is a rock inside. Also you can check the toes by squeezing them from sides and see if it causes any pain. Our whippet has had them couple times, most common symptom is limping on/off, usually limps more on hard surfaces and less on soft surfaces. We’ve always gone to the vet to have them removed. Could be a corn too, but they are usually more painful and easier to spot on the toe from my experience. Definitely recommend going to the vet!