@Shiroi_Kage He would be the one to eat it(and the spirits) and it would be embarrassing for a modern person especially Japanese much more let anyone to eat it to do not to mention Japan used to do that but due to some people suffering from parasites they discontinued. Even if there are no parasite the stigma is still there.
This is a guy that did not want to touch zombies to retrieve cores so he is the type that would not want to eat vegetables that has human feces on it. He also cherish the spirits so he would be uncomfortable letting them eat those.
Besides , the microbes needed are specifically beneficial ones and the one in human feces is not necessary the one needed and it might even cause harm as even if he is immune to it already , future non spirit residents would not be so immune. The above mention case is one since the victims are USA soldiers. Not a spoiler since they would surely add non spirit residents since it would be pointless to revive the dead lands since he appear not be immortal .Revival would end once he die of old age.