Children may come across as shy for many different reasons.
Some are shy because that’s the way they are. Some children model their parents’ shyness. Some children develop shyness as a result of teasing by family or friends. Sometimes shyness is a manifestation of a poor self image because of negative messages the child has been subjected to in his environment, or poor body image due to some real or perceived physical defect, causing him to withdraw into his own world.
If being shy does not bother him then it is not a problem and you should accept that he is just that way. If it is a problem for him then it needs to be addressed. Pressuring your child to open up will not help. The more you refer to him as the shy child or push him to make friends with other children, the more likely he is to resist and feel uncomfortable and inadequate.
Joining a drama class is a good way to help children overcome shyness. Assertiveness training is a more direct solution. If you choose this route, consult an experienced professional who will recognise and deal with a more serious underlying problem if it exists. The worst case scenario could be that your child’s behaviour is the result of depression. His teacher will also notice strange behaviour so rely on her to bring this to your attention. If depression is the case, then seek professional help immediately.
Shyness is only a problem for the child if he experiences it as a problem. Being able to enjoy your own company is a great attribute that many people lack.