Toilet Training for Preschool

My oldest son is three years old today! I can’t believe how quickly the years go by. Everyone told me it would happen, but there is no way to prepare for how fast they grow. In the fall he will be starting preschool. The only problem we are facing with this exciting event is that he has a long way to go in toilet training. He wears big boy under-ware at home, but he still has a lot of accidents. I am afraid they will not let him stay at school if he has as many accidents as he has at home.