why my cat miaow when he poop?

Just wonder if his diet is OK and there is no problem with constipation.
He eats mostly kitecat, chicken hearts cooked an drink water with milk.
Looks like everytime he wants to defecate he is in pain, otherwise why he would miaow? cat is 8 months old.
thanks

He may be constipated. Does he sound like he's in pain, or just like he wants to make some noise? I know that baby kittens usually meow a lot when they potty in the litterbox, but usually they outgrow it. Milk isn't good for adult cats, so possibly that's causing some tummy trouble for him.

Are you sure hes not peeing/trying to pee?(could be infection or partially blocked)
Are you seeing pee and poop on daily basis?

May want to cut the chicken hearts out and see if it settles on just cat food

Sounds like he is straining to poop, a sign of constipation.
Out of curiosity, is this milk lactose free, or milk for cats? He might have a problem with it, as most cats (the great majority of cats) are lactose intolerant - milk is not good for cats.
Also, what ratio of chicken hearts/cat food are you giving to him? He should not have more of 1/3 of his diet of chicken hearts, or anything other than cat food, otherwise you will have to balance his diet with other things.
Cat food is balanced for him.

Has he been to the vet?
Try giving him some canned pumpkin mixed with wet food every day, My cats eat 1 tablespoon a day, but they are adults.