I am not trying to get your hopes up, and I don't want you to have false hopes, because it sounds like it could be FIP. But so you are informed, the titre test tests for the corona virus, which can be a million things, it is NOT just FIP. And because they're carrying the corona virus does not mean they're sick with anything! When Tuxedo came down with something like Feline Hemolytic Anemia, our regular vet was on vacation and the vet we saw first thought "FIP" and got us into a panic, and we brought all the cats in for a titre test. Three of the five had VERY high titres, and the vet put them into quarantine rooms there until we picked them up. We were freaking that three of our kitties might die of FIP.
Then our regular vet came back from vacation, and explained that a high titre count does NOT mean FIP. There is no test for FIP. It can confirm other symptoms and say, yes, it is PROBABLE it is FIP, but that's the extent of it.
As it turns out, none of our cats (knock wood) have FIP. They may
be carrying the virus that can cause it - but even that there's no way to know for sure.
If we hadn't had that second opinion for Tuxedo, he would have died of anemia, not FIP. As it is, thankfully we got a second opinion just by default of our regular vet being on vacation and seeing Tuxie when he got back from vacation. He is either smarter, more experienced, or better informed than the other vet that initially saw Tuxedo. But he looked at the exact same blood work results, and came up with a different opinion. With FIP, you do nothing except make Kitty as happy as possible. Anything else, and there is an active treatment - or at least choices to make.
Based on our experience, we'd always get a second opinion on a diagnosis like that, just to make sure.
for you and Mons. He has a loving home and caring meowmy, and that's what counts.