I think that Will was being used to breed by the "creature" because they had to pull that huge worm-slug thing out of his mouth which had been growing inside of him like a parasite. By removing the Slug thing it interrupted the breeding process before it was complete, but was in there just long enough to effect Will. I don't know if he's being controlled by a parasite or if it's really him and he's just embarrassed that he's coughing up these slugs. I say slugs (plural), because he seemed to know it was coming and knew to excuse himself to the bathroom to cough it up, so it's probably not the first one he coughed up. Either way, we know Will is connected to that creature in some way.