Maybe because he is not very used to facing people outside his family and school. Many a times if we do not let our children interact with the outside world this type of shyness develops.

Best thing to do will be try to take him to social gathering where there are other kids of his age. I am sure this will help.

It will be a good idea to get your child enrolled for some hobby classes there he will get to interact with different people and this way he can get out of his shyness.

My daughter behaved similarly I will tell you what I did. Some of the things which helped her are
1. Regular visits to my friend's place who had kids of her age. There she got a chance to express between kids of her age.
2. Enrolled her for a painting class which she loved. She got a chance to express herself through paintings and built her confidence as her work got appreciated.
3. Encouraged her to participate in more extra curricular activities at school and not worry about failures.

Hope some of these steps helps you.

Sometimes unknowingly we adults are responsible for our kid's behavior. I can tell this because this was the case with me. I am an introvert person and used to have few friends and I talked a lot to them only. Seeing me my kids also started having similar attitude. I marked that my son was very quite in public though he used to be smart at home and very talkative.

On discussing about his behavior with my husband he pointed out that he was so because he found me doing so. Since then I have been trying my best to be more friendly in public and with it I have seen changes in my son also.

kids behaviour change very fast. today they are shy. it is possible that few months from now you will be thinking why doesn't he respect people around him little more
don't be woried and at least don't show your concern to your son. encourage him in whatever he does.