My DD has watery/runny/bitty/slimy poop like that when she's teething (acidic saliva) or has a cold (snot) or is on antibiotics (general tummy yuck). Are any of those applicable to your LO? How's his appetite?
If he's having watery poops, make sure he's drinking enough and is well hydrated...if you suspect he's not, get him to the doc asap. Also if it lasts longer than a couple of days, or seems otherwise ill.