My son also has autism
It may take some time but I always reassure my son and I started to time or watch for certain signs he would have to use the toilet and then I would take him. My son only wants to be in the bathroom only when he has to use it so bringing to before hand would make him cry especially if it was a public restroom
It sounds weird, but my son loves things that sink and float so we did a container and he played with that concept for a week. Then I encouraged him by seeing if he could get his poop to float 🤷🏼♀️
How old is he