However, since these children are physically attacking your child, it is important for you to talk to the teacher to get the full story of what is happening and work together on devising a solution. First of all, he should have a planned verbal response for when these boys or other children bully him. If this fails to stop the bullying, he can shout at them. This assertive approach usually works because it surprises bullies and draws attention to their actions. Your son needs to develop some solid friendships with his classmates.
Bullies are reluctant to attack children who have defenders.
Since he is so shy, he will probably need your help. Invite a classmate over to play. It will definitely be easier for your son to get to know another child individually rather than in the classroom situation. Consider the idea of inviting one of the children who is actually bullying your son. Often, a bully is a child who is also being bullied by others, and in return picks on a weaker or shyer child like your son.