Tough one. It would seem that you introduced the element of medical risk and liability by bringing up the "may need treatment" issue when asking about outlets. The airline noted that this could not be accommodated and put it in your hands to eliminate their risk of liability with a Drs note for your son to fly. Not sure any compensation would be due.

Put it in a different perspective:
Passenger: My son has a peanut allergy.. its not so bad but he could die if anyone consumes peanuts...

Airline: We cannot guarantee a peanut free plane.

Passenger: Ok...we will just take our chances..

Airline: We would like a note from your physician saying it is ok to fly

Kind of reasonable in my view