Mc already immune, aside from being trained in the nrt stuffs by his dad, 
he already sacrifice his life for the spirits, he already accept that he will not stay forever with them but will still support them to the best of himself. And thats why
Theory: he will find a solution to bring them back at the end but he cant bring himself back along with them since his body/life belongs to the spirits, his existence already belongs to that world and not anymore to the earth.