over prescribed antibiotics with side effects

My 14 yo son was recently given augmentin xr 2000mg 2x per day.  I stopped it after 5 days as he was complaining of extreme stomach pain and headache.  He also c/o being very hot but did not have a fever, actually his temp was low 96.5.  Stomach pain and headache continued for 2 weeks but intensity varied between the 2.  Was started on Prevacid and after 1 1/2 weeks felt better.   Had Cat scan of stomach with mild splenomegaly,  MD said not to worry but RBC, hemoglobin, hematocrit and creatinine levels were all elevated as well.  Who should I get a second opinion from?