Sorry to hear he's got a temperature
I'm sure, as the vet said, its just to rule out FIP - but there is no specific test for FIP itself - it will just be a test to see if there are antibodies to coronavirus (the virus that can, in some cases, cause FIP). Even if he is positive for coronavirus (and many cats carry the virus without any ill effects), it still doesnt automatically mean FIP though.
I had all this with Gwynnie, they were always moving towards a diagnosis of FIP because she had high coronavirus titres and because of the absence of another diagnosis, but she didnt develop FIP.