I haven't worked with autistic children professionally, but I think it important that any child understand and agree to the purpose of the medication or health procedure.
--> If a child doesn't want to eliminate (for whatever reason) s/he won't.
Autism makes this more complicated, but the bottom line is that one should not treat an autistic child (or any child) as an idiot.
I don't know much about autism, but I understand it as a focus issue -- too much focus on one aspect of life.
If the autistic child can be brought to realize that they need to eliminate regularly, one might succeed with treatments that had otherwise failed.
Best wishes! I hope your child brings something new and wonderful to the world.