Depends. If basically nobody knows he's vulnerable to fire, it's a +2 total catch. If some people could figure it out with some research, it's a +3 total catch. If it's just obvious ("Hi, I'm a water elemental!"), that would offer a +4 total catch... except that catch value can't exceed points spent on toughness+recovery, so he'd only get +3 worth of benefit. (But could still get the +4 later if he bought toughness or recovery up to supernatural.)
Edit: just to clarify, the "some people could figure it out with some research" level has two sides; there is "He's clearly an X, but we'll have to do some research to figure out what Xs are vulnerable to", or there's the reason most fae have a +3 catch for cold iron: yeah, everyone knows fae are vulnerable to cold iron - what takes research is figuring out if you're looking at an actual fae (there are so many types!) or some other sort of nevernever beasty that'll just laugh at you if you try to hit it with a crowbar.