health insurance claims help!

My son has bluecross HMO and he saw his Primary Care Doctor for regular check up Oct 2004. But the hospital billed insurance ($170) with the correct doctor name and wrong Tax ID at that time. Insurance requested correct TaxID at that time. Somehow hospital didn't response until Jan 2006. Now the insurance reject to pay bacause of hospital's too late response. The hospital asked us to pay and otherwise our credit will be affected. I don't under such situation, where can we get help? Although the amount is not big, we are just angry.