This is just heart-wrenching!
I don't have any idea what the legal system is like in the Netherlands... but are you sure you have no say? Here in the U.S., it depends on which State you're in, but unless you're divorce specified that she has full legal custody... you should have some recourse. We have something called "joint custody" which means both parents must agree on certain decisions. Can you at least get a second opinion from another doctor to dispute the schools insistence on drugging your child? or try another route... deal directly with the school... do they REALLY have the right to ban your child if you do not agree to drug him? Can you get some legal advise? Start researching everything about your rights and what the school system can and cannot dictate to you in regards to your child's health status.
Research and start writing letters! And document what you find to help others in this same situation. Who knows? Maybe you can use this to dispute her custodial status and get custody turned over to you?
I can't believe this is a global phenomenon!!! I thought we in the U.S. were the only ones idiotic enough to drug an entire generation or two!!
Peace