My DD1 was a painfully shy, nervous, insecure toddler. Her most frequent response to any question at 3, 4 and 5 was "I don't know" almost invariably followed by the answer. Now at 16 she is relaxed, confident, self-assured and optimistic. ime you need to accept that he struggles with some situations, talk him through scenarios, teach him creative ways to deal with things he finds difficult. What he needs is power, and he probably feels that he has none at the moment.
You need to ascertain the form taken by the bullying to work out whether he can in fact "handle" it alone using coping strategies. Sometimes doing things for them, however much they are finding it difficult, doesn't actually help them all that much. If the bullying is a bit of name-calling or being excluded from games, tell him that the children are being unkind but that they don't realise it, that they are too immature to realise it yet (and in truth, they are, however older/more worldly they appear to your DS). Give him strategies on how to deal with name-calling (ie not rising to it, or responses he could give that would deflect situations), and how to ask if he can join a game. If he tries this and it doesn't work (maybe try to get the school on side during this phase to observe how things are going without interfering), maybe then move on to more active adult involvement. At all times underline to him that yes, it is hard, it shouldn't happen but that he can get stronger and learn to overcome it. Tell him you love him and bolster his confidence at home a lot and make sure you ask him delicate probing questions as often as possible about what's happening at school.
If the bullying is physical or involves large gangs of children, get the school involved straight away. A child should not have to fear for their personal safety at school.
I have been there with all three of my older children in very different ways for each of them and it is heartbreaking to see them unhappy at school, but so gratifying when they learn to deal with reverses of fortune. I wish your DS luck with finding his inner confidence. He WILL!