The “flask” in question is a largeish water skin that is mostly empty save for a trickle of something sloshing around inside of it. He has little more than one swallow left. Any longer and he’ll need to resort to finding a donor. Maybe Reigen will volunteer. He seems uncommonly committed to caring for a stranger he’s just met.

Joscelin reaches a small hand for the flask and has the presence of mind to say:

“It’s...from the train. The last of m-my medicine.”

When is the train coming? He’s weak as a kitten right now, but still strong enough to drink from someone and then erase their memory of the attack if he needs to. He doesn’t want to. Not even when he’s like this.