I can share with you my experiences so far and I'm sure others can add any experiences they've had. The 4 month old kitten I received was very bony but had a big tummy and I thought she just needed to fill out from having just been spayed. She ended up dying 2 weeks after I received her. The tests never did absolutely confirm positive FIP I don't believe and the breeder had extensive testing done on her too. So unfortunately testing will not rule in or out positively FIP exists or can develop. Her other symptoms were lethargy/depression, not eating well, continual low grade fever, always sleeping, and the bony backbone and hips but with a tummy. Since then at about 5 1/2 weeks her sister has died from it (breeder still had her). She stopped gaining weight and wasn't eating properly (only symptoms I'm aware of, could have been more). I have one of the brothers (was a replacement when the first kitten died), and he is growing and gaining weight, very active, big eater. But I watch him very closely because he could develop FIP at any time during his first couple of years. I also have the mother who will be 2 years in April and I watch her too. She's very active, eats too well (overweight), and healthy looking. According to what I've read, cats that have come in contact with FECV (which can develop into FIP, but not necessarily) are more likely to develop it during their first couple of years (actually about 18 months) OR when they are in their ederly years. It has to do with lower immune systems not being able to fight as well, thus allowing the development of FECV to FIP. I'm already attached to my 2 kitties and don't want to give them back to the breeder, so I've decided to just take one day at a time and hope they at least live 10 years or so. I'll be happy if the kitten who is now 7 months old makes it to an adult and lives at least a good 10 years.
I agree with you about your older cat already being exposed, but others may not agree. I haven't tried separating mine at all. After the first kitten died I did wash all the bedding and totally changed out the litter and sanitized everything. I keep a very clean litter box because of this and also am diligent about keeping their food bowls very clean and use a fresh flow water fountain to encourage them to drink more water.
You may want to get a 2nd opinion on your kitty, I think I would if I were you. If he hasn't shown any of the classic signs of FIP it could be something else. You didn't mention any symptoms, is he having any? Anything I've read, you have to really go with definite signs of FIP along with any testing before you can classify it as being an actual FIP case.
Anyway, that's the extent of my experiences so far. Before this I didn't even know this bad disease existed!
I wish you lots of luck with your kitty and hope he turns out ok!