What can an overdose of normal saline do to an eight year old who is already hydrated?

my son's i.v. was not clamped and he had way too much normal saline thru his i.v. site and became unresponsive and developed brains swelling and his potium level was low due to diluting his electrolytes. a million tests were done to find out why he declined in health and all doctors are telling me the saline incident had nothing to do with it but i believe it did.