Hind Leg/Paw Injury

My dog has not been using his hind leg other than momentarily placing it on the ground. Since there was no visible damage, we thought that he had simply stepped on something (because we were outside when he was walking an suddenly yelped) and it had only punctured, but again, there was no bleeding or visible damage. It has been about three weeks and he still hasn't gotten better. What do you think it is and what do I do?