3 y/o leg pain early one morning. ibuprofen, ice, elevation, distraction didn't help and eventually we went to er. 3 days later it was the next leg, then an arm. he has had countless blood tests, x-rays, swabs for viruses. no diagnosis. please help?

If possible a university clinic might be best for diagnosis and treatment. Yes diagnosis very important

Continue to monitor temperature and blood markers of inflammation and infection. Did your son have a recent infection or COVID-19 in the past? Watch for signs of fever and poor growth. Musculoskeletal pain may shift and improve with rest and local heat. Avoid use of NSAIDS if cause not known or no relief.