Toilet training can be a challenge in children with an autism spectrum disorder. These children have difficulty with the speech and language, communication, social engagement, as well as wanting routing, and tend to be preservative and at times rigid. So, all these three core characteristics and features can come in the way. In order to be toilet-trained, you need to have a word for it. You need to have the awareness of it, and being speech-delayed or not having the language can be an impediment. So finding a word, getting the child familiar, once they have a few word vocabulary, whether you want to use "poop", or "toilet time" or whatever language and whatever lingo you want, find a consistent word. When the child has a dirty diaper, you need to say it.
Have them repeat it if possible. You also need to pick up on some of the cues that your child will give you before they have a bowel movement. For example, even in their diaper, you see a change in expression, whether the face gets flushed or they're going into the corner. If you can use that time to get them to come on to a toilet seat, it would be helpful. Having them watch and monitor other people using the toilet. See if they will do it through imitation, getting them a little toilet seat. I mean a lot of these you use for regularly developing kids, and you can use the same strategies.
With kids with an autism spectrum disorder, you need to be a little bit more sensitive to their sensitivity, which may be sensory. They may not like the sound of the flush. They may not like the texture of the seat they're on. They may get comfort by actually wanting to have the bowel movement in the diaper, because they're used to that sensation and that proximity, so doing things on a gradient and actually analyzing what the issues are with your particular child.
One of the tips that I do find helpful and parent's use is something known as the 'gastrocolic reflex'. They're generally after a meal. When there is the gastrointestinal system active, soon after you have a bowel movement, and watching your child, for what is their routine, when in their day do they end up having a bowel movement, you can use that time. I've had kids hold on to their bowel movement as a control issue, and you see that happening to the point of stool leaking out and it becoming something we call 'encopresis'. And for that, you need to speak to your primary doctor because stool may have been held on to and gotten constipated and impacted. Those kids need medical care and medical intervention of laxatives and stool softeners, etc.
But routine toilet training, as you develop language, as there's flexibility, as there's change in routine, and a lot of kids just one day decide, watching somebody else, that "I want to use the toilet." I've had cases where kids have been toilet-trained overnight. And for some kids, it becomes an issue of sensitivity to the much older, and you have to work with you child involving not just the pediatrician to rule out medical issues, but possibly the occupational therapist and the psychologist if there's a behavioral beast with. There are many strategies out there. You can choose which one works for your child, and share it.