What Leads To Extreme Pain And Fever After Undergoing Knee Surgery?

My son is 11 had knee surgery came out screaming in pain spent three days in the hospital. I asked for tests to figure out why they said he just can t handle pain. I kept saying no something is wrong . I had his primary doc to run a sed and ferritin and RA factor and some other tests .( I have auto immune disorder) sed came back 62 ferritin came back very high . He had 100 fever with all over severe pain he threw and had stomach pain so they put him on Zofran. Hhis ortho said sed rate is no big deal and won t run tests. Something is very very wrong. He still can t walk three weeks post surgery he was supposed to walk two days after