Sadit ~ I have a quieter child too, and I do completely understand how you feel. I feel the society we will live in today glorifies the type of personality that puts themselves 'out there' and seems to not celebrate the more studious, quieter and sensitive at all anymore. It is with real disdain that we find ourselves only celebrating the the extrovert when actually the introvert has so much to give to the world, their words are both incredibly considered and thoughtful. Traditionally in the UK we have always preferred the quiet elegance and the considered, we seem to have been brainwashed by american culture ~ the brash and the bold has ploughed through our old values.
However there are ways to fight back, and it starts with you completely accepting your child as he is, and not trying to 'improve' him in any way (I am not suggesting you would, but there is always a temptation if you think it will help him). It is not necessary for him to be anything other than what he is.
Is he quietly confident? Is he resilient?
If others are treating him badly he needs strategies to deal with them (that are in line with who he is as a person) school is a harsh place to be even for the most gregarious and resilient ~ it is not an ideal place to be if you are gentle by nature, however being gentle AND strong is a very powerful combination. Come up between you what is acceptable, what isn't, how to deal with unkindness, and how to remain yourself even if it is tough in the playground, he won't always be at school and in this situation. Help him to identify like minded souls and invite them over, or out for the afternoon. There must be other children that are similar, find out who they are and make in roads into getting them together. If he has a small group of like minded fellows this will instantly boost his confidence and happiness. He seems to me like he hasn't found his group yet, and you can help with this. Run through the class list, and then make a plan.
Make sure he cultivates friendships outside school, and hobbies that he genuinely enjoys will all be good for his soul. Celebrate other quieter well known or famous people, so he feels he is worthy and that it does not hinder. Research relentlessly the great and the good that did not shout endlessly about their own achievements, but simply went out in the world and did them.
I do believe he will find his way, with a little help and guidance, celebrate his gentle soul ~ he will make a wonderful husband, father and friend one day when is grown up ~ he will be infinitely more employable and well liked....school is just a stepping stone.