Possibly the mom* was not in great shape either and thus not able to produce all the nutrients he needed.

They mention he had persistent diarrhea, which could also be pretty detrimental without things like electrolyte solutions and antibiotics a child in a first world country would receive.

* Maybe the buffalo? Article doesn't really say if it's cattle or carton.

I suspect the serving vessel for the buffalo milk may have been contaminated. Running water to wash dishes with isn't the easiest thing to find out there.

Mom may have been malnourished as well.