there are two common condition that must be screened for in this case. first be sure the baby does not have infant onset diabetes mellitus
(fasting blood sugar) or diabetes insipidus
(lack of anti diuretic
hormone). Urine electrolytes and serum electrolytes could be very useful. Visit a pediatric endocrinologist
or suggest to your physician these very likely conditions be screened for.
Thanks and hope this helps,