Gray stool can be caused by a few factors. In many cases, a baby’s stool changes colors because of dyes in his food or a change in diet . In rare cases, gray stool is a sign that there’s a problem with the baby’s liver. For instance, this change in color may be caused by a serious childhood liver disease known as biliary atresia , which is an obstruction of the bile duct and this is evident usually at the 2nd week of life with apparent jaundice . to exclude this problem we need to do basic blood tests : total and direct biliruben , GGT . and for the failure to thrive problem I need to know how was the baby s birth weight ? and is he a premature baby ? how much milk does the baby consume each day , by that we can calculate the caloric intake and increase it accordingly . I d like to do a thyroid function test and urine analysis and culture in which both thyroid disorders and UTI can cause failure to gain weight . I hope this helps