these are the methods I used to help my son:
1. I greet the person loudly myself
2. I told my son if he was too shy to say hello, then use gestures.
3. I praised him when he tried to greet the person, even in a very small voice, but I reminded him that if he said it loudly it would be even better.
4. I acknowledged his feeling of shyness, and told him that he would feel less shy after practising more.