I'm not sure how Candian law equates with the US in terms of FAPE, IDEA, LRE, etc but I would say something along the lines of
"If he poops in his pants he will have to miss the remainder of that school day. If he wanders off he will miss the remainder of the school day. That is not providing FAPE. You're the ones under a federal mandate to educate him. How you choose to do so is up to you but if he cannot attend school regularly he will be entitled to homebound tutoring in order to get his education. This will be extremely expensive - especially given _______ spec Ed needs. It also would not be meeting the requirements of LRE. But if you think that is a better solution than the no-cost option of having the teachers remind him to use the toilet I guess that's up to you."
If the principal or IEP team won't listen go to the superintendent then to the chair of your Board of Ed or equivalent. When speaking to them I would also point out the liability they are assuming now that you have documented in writing that your son is a "flight risk" and the school has declined to implement any plan to keep him safe. I think you can be pretty certain it will be addressed promptly.
This strategy worked well for us. Very well in fact.
Good luck!
