Beloved Girl,
Natto contains thrombolytic enzymes that help dissolve blood clots. One of these beneficial enzymes called natto kinase is formed by the natto yeast, so eating unfermented soybeans would not have the desired effect. I believe there are natto extracts (supplements) that might be more palatable to your father. Natto smells like horse dung, and most foreigners and half the Japanese can't stand it either. But if your father will do anything for his health, natto is often available in the frozen food section of Asian grocery stores around the world because it freezes well. If you can read Japanese, try to get the ones that say Nioi hikaeme, which means low-odor. Natto is relatively inexpensive. If your father dislikes the slimy texture, he might try eating it frozen, or another thing that Japanese do is to mix in some grated daikon radish which makes the natto more watery and less sticky.
Mari