I have a ?for tu all?what would tu do if tu had a son with your ex?

ok all I have a son with my ex and hes denying hes the dad because I left him in fl cause he was being Verbally abusive.And nows he's moved on with another chick and he's planning on having a family with her,he's denying our son's existance what would tu do in my situation?And I wonder what brc would do to lol?