How can I help my son to develop self-confidence?

Our son is only 4 ½ years old and is a very gentle, but energetic little boy.  He loves to play with pokemon figures and is currently in love with super heroes such as spiderman and ironman.  For his preschool, he goes to a local Montessori child care, so the kids who attend his pre-school are between 3-5 years old.  Since there is not Montessori school on PEI (this is the smallest province of Canada), my son will be attending a public school next September.  He will be starting elementary school; i.e., kindergarten in a French immersion program.

I am not certain whether is because he used to go to a small home-care (usually 4-6 kids per day) until he was about 3 years old or because of his genetics (from my side of the family), but he lacks a little bit of self-confidence.  Some kids at the day care push or tell him names, and he doesn’t say anything.  He is very quiet at the day-care and rarely raising his voice (of course, this is a little bit different at home!).  We keep telling him that he needs to be more assertive (of course, we don’t necessarily use this word) and has to learn to say things such as “I don’t like when you push me or call me poop”, he just doesn’t want to say anything.  As a result, some kids are always picking on him.  My husband and I are continually telling him how important it is to be able to stick up for ourselves.  Based on his behaviour and explanations, it appears to me that he feels that if he says something the kids will not play with him or will not like him, or simply will not chose him for games. Now, I remember that when I was young, I was exactly like him.  I was scared of everything (my son is very scared of animals even though he seems to love them) and had few friends (sometimes no friends).  I was just too shy to say something to kids who would make fun of me.  Now, in my case, there were other issues that may have explained my behaviour (unstable family and abuse) I managed to survive those years, but it was not fun. I don’t want my son to face the same problems or insecurities that I had when I was young, so I want to find tips or ways to help him.  What can I help him?