S.,
Okay, here's the educator coming out in me...what has changed in your child's routine at home? Has he always wet himself when he's with you guys at home? Have you asked his school teacher for the techniques they use to get him to go on his own?
Just a few questions.
I started out assisting in pre-schools when I was in college, and this was not unusual with kids who had a major change to their routine. For example, new baby, move homes, loss of relative, new addition of activity in sibling routine or illness of family or child.
I understand it's frustrating, but if he's never gone on his own with your then there is an issue with his need to be attended to. If you get angry and frustrated he knows that he's going to get a response from your...whether positive or not. What does your time with your kids look like? Do you ever spend one-on-one time with them? Is there family time when you guys play games or go to the park? (just trying to narrow it down)
My one stand out experience as a helper in preschool, was a little girl who was amazing at school and her parents always talked about how awful she was on the weekend or when they picked her up from school...it was so confusing, because she was so bright and they said she reverted to infant like behavior at home...sucking her thumb, crying at night and refused to sleep anywhere but, her old bed. After they took her to therapy, I think she was four at the time, they found out she had fears they didn't know about...dark, the loud noises the heater made, the toilet flushed different at home than at school...
Have you talked to him about it? I mean, sat down in his room or outside and asked him why he wets himself at home? My son is only 2 years old and not yet potty taught, but I think talking to him is helping us get through some very difficult times...visitation and routine changes. Maybe he is seeking some genuine Mommy time. We get our kids so used to us being the main caregiver, and then it switches so dramatically when they start preschool or we have to go back to work. Maybe plan an outing for just the two of you, and take that time to reassure him you're there for him whenever he needs you. Kids need constant reassurance of this, and it's hard once they become more independent to remember that cuddles and love can do wonders!
I hope this rambling helps a bit...good luck!
Deanna