Since your son can't think of anything to write about, he should be encouraged to describe personal experiences -- they are the easiest topics for young children to handle. At home, talk over possible story ideas with him, such as giving the dog a bath and visiting the zoo. If he doesn't have an idea for a story at school, his teacher can assign one or give him a picture to describe.
While having a topic will help your son, he probably also needs to learn how to expand it into a story. At school, the teacher should be teaching the students how to tell a simple story in chronological order. However, you can really help your son with this phase of writing by reading stories to him. At first, you should just ask him what happened on a page, then you can gradually have him retell more and more of the story. This will teach him how to organize what he writes.
Telling stories to your son is another way to show him how stories are organized. He will especially love to hear stories of your childhood. Also, every time you ask him to describe what happened at school or at a friend's house, you are giving him experience in putting a story together.
One way that both you and the teacher can encourage your son's writing is by having him share what he has created with a few friends. It will actually motivate him to write.