12 is quite young for a cat now and I would be having him checked out by a vet.
Do you know why his sight is deteriating cos at 12 I think there maybe a reason that the vet can help.
I have two birmans aged 15 and they are fine,,,,,,,,,,well franta maybe quite deaf but it seems to depend
Kocka went to the Bridge at 20 but was fine at 12 and didnt start having probs till she was about 16.
Every cat is different of course but 12 is not that old any more