I agree with Gill, vets are often too quick to mention the possibility of FIP, so try not to worry too much. Has your vet said he wants to do tests? Don't forget that even if Louis tests positive for coronavirus it doesnt mean he will develop FIP.
My Gwynnie was very ill when she was about 6months old and tested positive with coronavirus titres of 2,500 and my vet was pretty sure it was FIP (she also had other symptoms, sneezing, diarrhea, and lost the use of back legs - although that might not have been connected, we'll never really know) - anyway, she's two and half years old now and as healthy as they come!