Smaller poop is most often a sign of gas, chins can't fart (they pass gas as they poop) I would try some infant gas drops and see if that helps. If he has a gas bubble that can also make him not want to eat. Get some infant gas drops, you want 20mg of Simethicone per dose, which is normally around 0.3ml (you can't really overdose, excess is just peed out), every 8-12 hours. If he doesn't seem to be improving within about 12 hours then I would get him to the vet. If he stops pooping all together, even before 12 hours, get him to the vet right away.