He is not a dead beat dad

What is the name of your state? md.

what law is there to protect non custodial parents from having their character slandered by custodial parent ( ex. You father isn't doing anything to support you. He never has.)

There is no law that governs this. The court, however, may see it as Parental Alienation Syndrome (PAS) and the court REALLY doesn't like it. When taken to absolute extremes it can sometimes be grounds for a custody modification.