do i have a case? how should i proceed? is this a case against insurance or is this medical malpractise?

My son was just diagnosed with a malignant tumor called rhabdomyosarcoma. But the thing is he was born with this large protruding mass on his back in which the delivery pediatrician had told me he has never seen before in his entire career. feeling scared and concerned i asked expressed my concerns to the family pediatrician. At first i was told birth trauma, then free fluid. at 3 months old they did an ultra sound. it was not the imaging test needed to tell anything. they needed an MRI. however, i was told insurance denied it because it was non life threatening. i continued pressing at every appointment untill at age one and a half they referred him to a pediatric surgeon who does another ultra sound then proceeds to do an MRI which came back concerning. He referred us to an orthopedic surgeon who did a biopsy. Im just so upset that they did not do what was necessary to find this sooner. Malignancy is the tendency of a medical condition to become progressively worse. growing all along