I didn't know this disease is so present in the breed
It doesn't seem easy for a clear diagnosis, I think I should talk with my vet about this. . . .
Do you think a regular vet can give a good diagnosis for him at his age 3.5 ?
Unfortunately, there is no way to test for familial amyloidosis. They say the cats are healthy and have normal blood test results until they suddenly fall ill. I saw this myself. I had two modern Siamese girls who seemed perfectly okay until they were 6 years old. They were half sisters. One died in January, and the necropsy showed it was amyloidosis. When the first one fell ill, I brought her half sister to the vet clinic for testing. The half sister had a complete blood panel with liver function tests. She tested completely normal. But 9 months later, the "normal" girl fell ill. After she also died, I had another necropsy done. It was also amyloidosis.
There are only two ways to predict familial amyloidosis, and both ways are not very reliable. The first way is by the cat's health. Any cat that has a lot of inflammatory illnesses and infections, such as sinusitis, has a higher than average risk of eventually developing amyloidosis. The other way is to look at the pedigree and show the pedigree to experienced breeders in the Siamese and Oriental community. They can tell you if the lineages in the pedigree have produced cats that have FA.
The trouble is, many breeders have not done necropsies on their cats that have died young. Even in supposedly "healthy" lineages, FA may still be present. It is also possible that inbreeding all by itself causes at least some cases of amyloidosis. That is because it can cause a weak immune system, and weak immune systems can mean lots of infections. Any cat that has a lot of infections during its life is at risk of developing amyloidosis. It's comparable to the way brain injuries can bring on dementia in humans. It can be genetic, but it can also be the result of chronic infections and illness.
I think the best way to prevent FA is to buy breeding cats from experienced breeders who have a repuation for being very open and honest about problems in their lineages. Honest and knowledgable breeders are a better source of Orientals and Siamese than breeders whose cats win at cat shows all the time. As we say in PREOSSIA, "The most beautiful cat is a healthy cat."