I have no criminal record and am being called a pedophile, what can I do?

An 18 year old is with a 13 year old girl.Mother is fine with it, Dad no.Young man verbally being accused as a pedophile because Dad is angry.No involved at all.mother always with them in public maybe 2 times monthly.Dad has told his lawyer,a family counselor,his fam. & friends that this young man is a pedophile.He is not this, how can he clear his name so they will stop?It could harm his job, his college, ruin his name....Slander or can he just request to discuss with a higher authority?Mediation, a man to man talk in the company of police or what? If he was truly this...Dad or Fam. counselor should of had him in jail by now.