I think the section on "GM-Driven Compels" on YS 101 might be useful here. Basically, this is a case of the GM creating dramawhich the players don't have a say in. As any compel, there is room for negotiation, in theroy, but in the example you gave, there really isn't any middle ground. The players might have a choice to buy out of some of the consequences (in this case, that might mean that they find out about the condition early enough to help him avoid going all the way over the line into full vampire). But, to quote the next page:
Sometimes, it may seem as though there is no practical way to buy out of a “scenestarter” compel. Suppose you have the aspect My Dear Brother, and the GM proposes a compel with, “Hey, so you find your brother beaten to a pulp and left on your doorstep, with a note that says ‘Now we know where you live’ on it.” It would be pretty lame to spend a fate point and suggest that it doesn’t even happen.
I think this example is very much the equivalent of what you have planned.
So what you might do is just cryptically hand the players a Fate point each, possibly accompanying it with a comment about how there seems to be something ...
different about the kid (perhaps do this a session or two later, to make the connection with the rescue a little less obvious). If any of them buy off the compel, give them the info about the cause of that feeling 'for free', otherwise they have to either investigate the situation or guess or ... whatever. And if they just take the Fate and ignore the situation, then perhaps that means they'll be getting more Fate a bit later, as the find the kid standing with a bloody mouth over a fresh corpse...