Talk with your son's teacher and school counselor. Explain what is going on with him and ask for their assistance. You may hear that the teacher sees none of your son's anxious behaviors at school, as your son may feel more comfortable expressing his concerns about school with you. Ask the teacher to help your son feel good about being in her classroom, assisting him with making friends on the playground and at lunch. The teacher can also help your son feel confident in his ability to do the school work.
The counselor can also help with your son's adjustment. She can offer support in the mornings by meeting your son at the door and walking him to his classroom if morning separations are difficult. The counselor can also give your son some individual time or include him in a small group to help build his self-confidence.