All of it. However, it is more than just plunking down a piece of raw meat. They need the proper balance of muscle meat, bone, and organ meat. Look up BARF and Prey Model - those are the two most popular diets. Most 'better' pet stores sell frozen prepared raw that comes in little nuggets or patties. You just that what you need for the day, and offer the thawed patties/nuggets. Try those. But you can add chicken wings and necks - with the bones. Cats (and dogs) can eat raw bones, it's the cooked ones that are dangerous as they splinter. You can get chicken livers at the grocery store. Chop up one and freeze the rest. Unless the raw tuna was previously frozen, limit that as it can be filled with bacteria. Don't feed him any meat you yourself would not eat - this is not a time to dispose of meat that is "off" or "too old". You of course need to follow the same precautions you use when preparing your own dinner with raw meat you are getting ready to cook. That means hand washing, sanitizing the cutting board, etc. And be certain to wash his dishes after each meal! Cats and dogs - all carnivores - are designed by Nature to eat raw meat, bones and all. Remember that lions do not cook their catch prior to eating it, LOL! Mine are fed mostly quality canned, but do get their slivers of raw that I am preparing for dinner. I do not feed them raw ground meets as those tend to be teeming with bacteria (much more than a piece of beef or chicken contains).
A vet visit wouldn't hurt. He may need his thyroid medication adjusted, or he could have a bad tooth.