HELP! I think my neighbor is ruining my son..?

My 4 year old son started Pre-K this year and we had him fully potty trained about 3 months before school started. Well, he has on several occasions pooped in his pants at school. The school will not change him if it is pooh, so I have to go to the school and change him. I'm not sure if he does it to see me, or if he is embarred to ask to, or what is going on. When I ask him why he does it, he says "i dont know", and trust me I have tried everything that I know to get him to tell me. Anyway, my neighbor said to give him prizes when he doesnt have an accident, she said thats what her daugters dr. told her to do. Her daughter has slight autism, so i dont know if that changes things(discipline, teaching, etc.)... She has taken it upon herself to meet us at the bus(same bus stop) and give him a prize for about the last month. I was not real thrilled about this, cause i didnt want my son to know that anyone else knew about the accidents incase its an embarrment issue. I should have said something earlier, but it seemed to be working and it really made my son happy to get a little prize when he got off the bus. Last week, she started cutting back, like instead of everyday, she would meet us every 2 days. and this week shes said she wont meet us at all. when she cut back last week, he started pooping in his pants again. So, what do i do now? i really need some advice. ANY HELPFUL suggestions would really be appreciated!!