Feeding a 4 year old

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So our 4 year old eats what we have for dinner every night. We had chicken stir fry tonight which he likes. he ate 75% of it then sat with a piece of chicken in his mouth for 20 minutes. Just kept chewing it but wouldn't swallow it at all. He does this all the time and we don't understand why. But then he throws a fit when we make him spit it out. Does anyone else have this problem? What do you do?