The change in food is the most likely reason. I'd quit the boiled chicken and let him get used to the one food alone first. Some runny stools here and there aren't something to worry too much over, unless he shows other signs of a problem too. Lack of his normal energy, looking sickly, etc.. If he has the runs consistently, talk to the vet first, but in general you'd give his stomach a chance to settle, then something like white rice boiled to a paste and add a little chicken in a day or so.
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