I dont think that you should be too worried or make a big fuss about it. I am a very shy or quiet person from a very young age. Shyness is part of my personality, and I have learned to accept it. The more I got out in public and took very small steps in socializing with strangers it did get a little better.
The last thing you should do is to make him constantly aware of his shyness, and to say to him that it is wrong. Boost his confidence by letting him do things by himself. Next time you go to buy milk, take him with you and give him the money to pay at the till. Afterwards tell him what a great job he has done. It is very small things that can boost his confidence or break it. I now have a 2 year old son that is very extroverted, and will greet complete strangers, and he will make sure that he is heard.