That's really stretching it . Only let him do that if he shells out the points/feats/etc for poison-immunity, because it's silly and wouldn't work unless he's drastically the mass. Also, nerve-toxins like cyanide kill faster anyway. Not all poisons care about kidneys, especially in superhero-universe.
Have you worked out where exactly he's getting these powers? Even if it's tecnhobabble or "a wizard did it", that would help for finding a weakness.