Pitchounette, i feel for you i really do - it can be so hard sending your child into the care of others and not knowing what will happen.
I would definately go back and see the Teacher. You need to know what will happen if your ds soils in school time, what level of help will be given and that your ds gets a compassionate response to his medical condition.
Having a proper discussion about the fluid intake is important too, and maybe your ds needs a TA assisgned to him to get a proper routine sorted. Good luck, and if you feel that you are not getting anywhere go and see the Head.
Toileting issues in Infants is really common, and should be able to be sorted so that the child is happy, comfortable and clean - without a great palava !