My 4 Year Old Normally Very Active Son Is Crying

My 4 year old normally very active son is crying when he tries to stand or walk since yesterday evening. URI, low grade temp last Tuesday through Friday. No temp today, just continued cough. I am very concerned that he cannot bear weight in either leg. Went to ER today. CBC, ESR, CRP all WNL. X RAY ON lower extremities normal also. Sent us home to "watch him". What should I do?