My father has breakbone fever. What should I tell him?

He will be fine. If he is getting medical care. Most persons with "breakbone fever", aka dengue fever are not at risk, but just feel miserable with fever and deep aching pain. However a subset (interestingly, those who have had dengue before) can get really sick with vasccular leak and even overt bleeding. These patients need a hospital with IV fluids, blood transfusions, and medical expertise.
He will get better. Dengue fever, a mosquito-borne viral infection, usually has complete recovery, although in some persons it may take a prolonged time before the malaise (sense of feeling poorly), and fatigue may go away.