Testing positive for the antibody just means that at some point in his life, he was exposed to a cat with the coronovirus. The so-called FIP antibody is not specific to FIP and will show positive to any strain of the Feline Enteric Coronavirus. FIP is only a type of strain of it--most other strains are not a big deal.
From the Merck Veterinary Manual (http://www.merckvetmanual.com/mvm/in...bc/23304.htm):
"The virus is ubiquitous in cats, and many cats that recover from the infection remain carriers. Enteric coronavirus infection can be prevented only by minimizing exposure to infected cats and their feces. Cats with the enteric disease do not progress to develop clinical signs of FIP. Most cats develop an effective immune response on exposure and recover from infection."
If your cat isn't showing any symptoms of the disease, he doesn't have it. There are no accurate tests for FIP while the cat is still alive--the only official diagnosis is made through testing tissues samples, which are typically only taken through necropsy.