You need to get your son to a pediatric ortho. My Daughter also the same age, woke up one morning not being able to walk on her left leg. Very strange, and she would cry like it was hurting. Her father and I could not figure out what happened, because she had not been injured. We took her to the Er after several hours, she would not walk but crawl real strange. The ER doctor was an idiot, because by this point she had a feaver. He said she sprang her knee and has a soar throut(She had drank orange juice that morning). Anyways he said the sprang would get better and gave us an antibotic for the imaginary soar throut. This was on a Sat. That Monday, she still was not walking. I took her to her regular doctor and she sent us immediately to the ortho. Who explained that she propley had an infection in her Knee that was very serious. That two year olds do not have sprang, they're joints muscles are still to flexiable. That is had to be an infection. Which happens sometimes in young children, and if not treated could become very serious. He said for us to watch her because she was on the antibotic, but the antibotic may not completely stop the infectin. That it's possible that the infection could come back in a year, and then she would have to be hospitalized and hooked up to IVS. So for the past four months we've been watching her, to see if anything shows up. Of coarse we pray that it will not. I am not trying to scare you. but from my experience two years olds limping with out a fracture is not a good thing. If he is still doing it I would call the Ortho. Just to be on the safe side.