i say go "cold turkey"... fast him for a meal, then offer raw the next. cats can be notoriously picky though, and some take a while to want to eat a raw diet... others just dive right in though!
can you get little samples from your petstore? a few medallions of each? often an animal will be allergic to a cooked meat, but in raw form it's just fine, it's something you will only know by trial and error unfortunately. but to be safe yes do start with fish, then if he does well, maybe bison, beef, etc.
wishing you the best of luck!
