We had this and turned out to be cows milk allergy. At first i out it down to teething but when it was a poo after every feed for several weeks i was so stressed out wondering if i will be dealing with an explosion. Since being under the doctors/dietitian its so much better!
Likely a tummy bug, seems to be one going around
We had this and it she was lactose intolerant. I was check with GP
If you are concerned you might want to call your gp But if he’s teething might be it as teething sometimes causes diarrhea in babies