If there hasn't been any significant changes in diet and he's not consuming large quantities of green food/drink, it's probably some kind of infection even if he seems to be feeling well otherwise. I'd call your ped - they'll probably ask for a stool sample (you might need to get a special container for it) so they can determine if it's something that needs to be treated.
__________________
To be yourself in a world that is constantly trying to make you something else is the greatest accomplishment ~ {Ralph Waldo Emerson}