Natto is fermented soy beans. It's very a healthy food, but it's pretty stinky and has a very nasty texture. I'm not bashing it, that's pretty much my observations of the stuff. A lot of medical research has backed up the supposed health benefits. I'm pretty sure anything with soybeans is good for you.

After smelling and tasting it, I believe the question to ask him is "Would he eat it?". Give it a try.