What Causes Extreme Lethargy In A Child?

my grandson, 3 next month, has the following symptoms: extreme lethargy (sleeps 15 hours), was anemic in mid-April and has taken meds & levels are better, diminished appetite, skin is very pale. All blood work has checked out okay, as well as chest xrays. His pediatrician is diligently looking for answers, but nothing yet. Could it be something in his home environment. He was very ill most of April with bronchial infection mixed with flu.