??? I dont know that liability insurance is really necessary. Seems like all the places I've been to ride made me sign a liability waiver that stated I understood that motorcycle riding is a dangerous sport and that I am participating at my own risk and if I feel the situation is dangerous, that I will report such conditions and not race/ride under such conditions.
Maybe he's worried about you/him riding on private or public property??? well, I'd think you would have to prove negligence or deliberate intent to injure someone. Seems like that would be his word against yours. Hard to prove and win I'd think...but i'm not a lawyer and NOT giving legal advice here. If it's not expensive, then why not let ur dad live with a little peace of mind??