N.G.
I work in retail, grocery actually. And someone once ate month old yougurt, and brought the container back to the store and we sent it off to be tested. It came back negative for all not normal yogurt bacteria, and I know gogurt has more preservatives than most, so I honestly think your son might have a touch of a flu going around. Food poisioning sets in about 12 hours after you eat the bad thing, and lasts 24-48 hours of nonstop diareeah and vomitting, so he is probably okay.
Good luck with that...
N.