I agree with the above two messages and think you are to be congratulated!  It is drummed into us to give extra hydro when are kids are ill and that it is better to be safe than sorry as giving extra will do them no harm in the short term and can potentially save their lives.  Your son certainly came to no harm as a result of your actions and although the doctor may say it was not needed, in my experience, they are not always right! I am a great believer in mother intuition and if you see your son suffering and feels he needs more hydro, trust your instincts - you know him better than anyone! 

As Carol says, what does it really matter that the tests were not done on that specific day, they can be rescheduled and will no doubt be more accurate when your son is in better shape anyway.  Once again, well done, in my opinion you did the right thing!