Let me preface this by saying that I'm not a veterinarian or nutritionist, but I did extensive research to help my own cat a few years ago.

It sounds like your cat might be suffering from food allergies/intolerances. Cats can become sensitive to many different ingredients -- artificial dyes, preservatives, wheat, corn, soy, dairy, potatoes, eggs, duck, pork, beef, fish, even (rarely) chicken and turkey. This is not a comprehensive list, but these are a few possible suspects.

My kitty developed a food intolerance after eating a premium brand of kibble (Blue Buffalo) for several years, when she was around the same age as your cat. MeowMix is a lower-quality food with many less-than-optimal "filler" ingredients, which will make it difficult to narrow down which particular ingredient is causing your cat's issues.

I ended up solving the problem by switching her from kibble to Fancy Feast Chicken Classic canned after trying several brands of kibble with no success. She was able to eat the canned without any problems, and has for the past several years. At first she was very hesitant because the texture of canned food is so much different from kibble. But, since she couldn't eat kibble any longer, hunger won out after a couple hours and she started eating canned as though she had been eating it her entire life. (Never withhold food from a cat for more than 18 hours -- this can cause a serious problem called hepatic lipidosis.)

You may also want to try adding a probiotic to his diet (personally, I like FortiFlora; you can also sprinkle this on canned food to entice him to eat), or giving him some plain low-fat and low-sugar Greek yogurt if he will eat it.

I also recommend that you switch your cat to scheduled feedings (2-3x per day) if he is free-fed. Additionally, make sure you're sticking to foods that are part of the normal cat diet. An outdoor cat eats small songbirds, rodents, and rabbits, so I feed my cats only chicken/turkey and rabbit foods. I keep fish to a minimum and never feed them venison/pork. If my cats couldn't theoretically hunt and kill the animal, they don't get to eat it!