Its potentially VERY serious for him. Its basically the feline version of HIV, and yes, there's a feline version of AIDS too which the FIV can turn into. So potentially, if he got sick, he could end up REALLY sick.....
On the other hand everything I'm reading, and after talking with several owners of these cats, its NOT highly contagious. The BIG vector is a bite deep enough to penetrate the blood stream. There MAY be a slight chance of a vector from a large amount of saliva, but it doesn't actually appear to be very large. Heck, kittens born to an FIV positive queen have a less than 50% chance of being FIV positive themselves! So as long as we're very carefull about intros, and he and Trouble get along well enough there's no fighting, the risk to Trouble appears to be minimal.
And there's still a possibility that the final bloodwork will come back negetive......
So everyone cross your fingers for me!