M.F.
I have a couple friends whose kids are like this. I think if he were pooping in the toilet it would be more formed, but since his tend to be loose anyway, once you squish them into a diaper it will naturally become a bigger mess.
The only thing I would be concerned about is some sort of digestive disorder like celiac disease, which is an intolerance to wheat gluten. It's fairly common and often undiagnosed. A trademark of celiac is slow weight gain and/or delayed growth. It should be easy to test for, so ask your pedi about it before you go in for the appointment in case they need you to save a diaper for a stool sample. While they're at it they may check for other things.
However, if he's growing and learning fine, it's likely that this is just his normal BM.