sounds like he might have a stomach virus. Its nothing to be concerned about just watch him and make sure he drinks alot pedilayte is best if hes not eating
DS got this last time he was breaking teeth. His poop was so acidic from all of his saliva, that it just burned his skin the second he pooped. It was so runny that he leaked out of almost every diaper for 3 days straight- something he hadn't done since he was a newbie.
We gave him some naked time, and put Desitin on after every diaper change. It stopped after a couple days.