m not going to be 100% positive on it, but if your son drank cranberry juice (lots of it) check into the effects of the oxalates(
in cranberries) and the calcium(
in your urine) building up to possibly form
kidney stones, the last question on the linked page will be helpful if that's the case.
http://www.uti-cleanse.com/faqs.php
If not, the doctors could be just concerning about kidney infection due to the bladder infection which can get very serious.