Hi, I'd like to help you with your question.
Usually, there is a cause for such symptoms. Was your son bullied in school? Was there stressful situations in the home? These are two of the most common causes for children developing anxiety reactions. You may have to talk with your son to find out if there was anything that caused him to start feeling this way.
There are some steps you can try to help your son:
Acknowledge his feelings- let him talk about how he feels and don't downplay anything he says by saying, You'll be ok or Don't worry. He needs to know that you understand him and that his feelings are important.
Tell your son he can always talk to you no matter what happens- he needs to know you are there for him and that he can talk about anything with you.
Let him express his feelings- even if he rages or cries, let him know it's ok to let it out. Often, tears can help the anxiety go down.
If you find that he is being bullied, then take action. Otherwise, let him handle himself. The more he perseveres, the more confidence he will develop. Don't give in to letting him stay home or resisting his activities.
Help him find things he is good at and encourage him to participate. Activities that increase self esteem such as karate or team sports such as soccer can help him develop confidence. Praise his abilities as much as you think is reasonable.
Have your child checked out by your family doctor. Be sure there is no physical cause for his anxiety. If none is found, ask the doctor for a referral to a child therapist. Your son may need to talk out his problems with a professional who can guide him through and help him develop ways of coping.
Here are some books that may help you as well. One is called Overcoming School Anxiety: How to Help Your Child Deal With Separation, Tests, Homework, Bullies, Math Phobia, and Other Worries by Diane Peters Mayer. A book for your son- What to Do When You Worry Too Much: A Kid's Guide to Overcoming Anxiety (What to Do Guides for Kids) by Dawn Huebner and Bonnie Matthews. Another one is Parent Talk: How to Talk to Your Children in Language That Builds Self-Esteem and Encourages Responsibility by Chick Moorman. You can find these on Amazon.com or your local library may have them for you.
I hope this has helped you,