17 month old weights 20lbs, alkaline phosphatas level of 2, 125, diarrhea x 19days, stools black not back to normal, ?

Failure to thrive: Your child's weight is below the standard curve for age, and with this prolonged diarrhea and black stools and severe increase in the alkaline phosphatase, I am very concerned and i think your baby might have severe vitamin d defficiency, anemia , and electrolytes disturbances that might put his life at risk. He needs full work up at least by a pediatrician or a pediatric gastroenterologist.