Hi Sheymama,
I know what you are going through, believe me. My son is almost 7 and has been fully potty trained only since he was five. It may take another year or so for your son as well. The direction that I would take is to meet with his teacher and discuss utilizing a VISUAL program. What I mean is, use an icon system (Picture Exchange System/ PECS) In the bathroom at school and at home- it is sooo important to be consistent and use exactly the same program so as not to confuse your son.We had posterboard and on it were pictures of each step of the training process, like pull down pants, pull down underwear, sit on toilet ETC. The icons or "pictures", if you will, were cartoon-like but realistic enough for him to understand the action that was expected of him. We also "time-trained" him. That means that we started taking him to the bathroom every 5 minutes and went throught he whole ritual, even if he didn't "go" and praised him ALOT. Then gradually increased the time alottment.It was very tedious but one day it all came together for him. Very happy day for me, believe me!
I am sure that I am missing key information. Maybe someone else can add to this, but my point is that my child (and yours too, I imagine) is a very visual learner. Using verbal cues at this early stage of the potty training, is not really helpful for them.
Good luck, I'm sure your son will be sucessful
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Leslie, Mom to Tristan 6 autistic and Lucas 8 NT