I hope this message finds you well. The permission letter should be notarized at the very least to be of any real legal significance. It should also clearly state, for clarity purposes if nothing else, that you son has a medical condition and what the medical is as well as how it is medicated. You need to have your name, address and probably two contact numbers on it as well (land line and cell line). While not necessary, a photocopy of your ID and your son's ID as well as a photocopy of his birth certificate could also be attach in an abundance of caution.
In short, I would rather send a letter giving medical power of decision over my child that is too thick, than have a hospital in Paris refuse treatment because the adult with the child does not have enough paperwork to prove who he or she is in the circumstance.
So, a quick list:
- Notarized signature and date
- disclosure of medical condition and relevant treatments for maintenance;
- your contact information, including name, address and two phone numbers (if possible);
- Not necessary, but maybe useful, photo copies of your ID, his ID and his birth certificate. (Be sure to redact any social security information though).
That should about do it.
Let me know if you have any additional questions or comments.
I wish you (and your son) the best going forward.