Sounds like he is going through a couple large transitions, potty training and separation. When my daughter had a hard time with separation the school worked with us to try and make up a plan. We had one care provider that we would do the separation process with every time so that she felt secure. You can talk to your son on the way to the preschool and give him an idea about what is going to happen (i.e., we're going to go to school today where you'll see your friends and teacher so and so. Once we get there, I'll give you a big goodbye hug and kiss and teacher so and so will help you say goodbye at the window. After snack time, I'll be back to pick you up.) Preschools are usually pretty adept at handling separations. My daughter really liked to visit the pet rat, look at the photographs of friends on the wall and give me a high five at the window. Not every separation was easy, but the consistency was important. As someone else recommended, I would talk with him to see if he has any fears about school or if something is happening that he needs to talk about and if he has any ideas that would make it more comfortable. My daughter has yet to do #2 in the potty, but I do know that doing this away from home is often very emotional for children. Hang in there though, I think this is a good opportunity for both of you.