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For how long has your son been urinating frequently?
From what you've written so far I agree with his pediatrician's next step.
I doubt this is diabetes mellitus, but your son may have a condition called diabetes insipidus in which the kidneys continue to make dilute urine even when the person is not drinking (and the kidneys should make more concetrated urine). If that's the case, this can be due to a kidney problem, or a problem in the pituitary gland in the brain that secretes anti-diuretic hormone. This urine measurement (and I assume some blood tests) are a good first step.
I hope this helps and please tell me if you have further questions.
Louise Sivak, M.D.
I agree with his pediatrician's first step.