Maybe he brought a parasite back with him, and it was that parasite that was causing him to vomit up a slug.
If that slug somehow grows up to become the monster that we’ve seen menacing Hawkins in Season 2 footage, then Will could have played a role in unleashing a monster, even if he didn’t do so intentionally.
At this point, it’s possible that Will’s experiences in that awful place have given him a connection that will allow him to warn his pals that something is coming; it’s also possible that he’s being controlled by something that didn’t let him go when he was pulled out of the Upside Down at the end of the first season.