E&O won't help you now as it may be a pre-x incident (depends on state and how policy is written)
But DO seek an attorney.. Are you in NY? Were you billing him on a %? If you were, mention this to your attorney, they in turn can make the decision to let the provider's attorney know that if this goes to court the provider could be sanctioned for entering into an illegal fee splitting arrangement.. this won't get you paid though if it goes to court as your contract will become null/void however it might be a bargaining chip since it's the doctor that broke the law NOT you. But last and not least and most important...LET YOUR ATTORNEY DECIDE THIS. And do not be shocked if you have to educate the lawyer on the fee splitting provisions, however NY is pretty clear so he should know about them. Call your state bar association for a referral if you don't have a lawyer. DO NOT DO PREPAID LEGAL. I know many billing companies burnt worse by using prepaid legal representation.