My oldest son is ADHD and possibly Aspergers. He did not train till he was almost 5 and once he made up his mind to do it, it was done. Now, he still wears pull ups at night and he is 6, will be 7 in April. I tried EVERYTHING....every training method but nothing made any difference to him. He cared not when he soiled himself. He liked helping me clean up pee off the floor. Our whole relationship became about potty training. Then my 2nd son was about to be born, he was being evaluated at the school district and qualified for the special needs preschool. Probably about 3-4 months after he started there, it was like a magic wand.....he was suddenly trained without me doing anything. You can't obsess over it. And it will happen eventually. Other than knowing he had some sensory issues at that point we didn't know that there was anything "wrong" with him.
If it is getting to the point that this is all you do/think about and you are having a hard time controlling your frustration in front of him, then I'd say drop it. There are enough battles to fight with our kids. And he will get it eventually, just it's gonna be on his time table, not yours.
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And...wow...Nora....you know NOTHING about Asperger kids. It has NOTHING to do with intelligence. It has to do with being in tune with your body and having physical control over the process. They understand, but they can't DO IT because their body isn't getting the right or strong enough sensory information. Plus, potty training is all about having control. Kids who have so much going on have very little control over anything in their lives. If they decide this isn't something they want to do, then that is a control mechanism. It has NOTHING TO DO WITH BEING SMART OR NOT. Most Aspie kids have a VERY high IQ, just not always alot of common sense. And personally, I do feel like you are putting the kids down by harping on the intelligence issue. Please educate yourself about what you are dealing with before you pass judgment.