Don't give him lots of bread, it won't cushion anything but may make him constipated which is what you don't want.
A plum pit contains around 9mg of cyanide, you need 200-300mg orally to get a fatal dose so unless he has eaten 20+ plum pits in one sitting you really don't need to worry about cyanide poisoning.
It'll pass in a day or two, no need to check his poo.
If he complains about tummy pain, has blood in his stool, starts vomiting, grows leaves etc, then speak to a doctor.