He always has had trouble relating to other children. He wants his way and gets angry when he doesn't get his way. When other kids figured out that he gets mad easily, then some make him mad just to get him in trouble.
My main concern is helping my child with his anger but I also need to know how I should respond to the children who try to bait him.
Unfortunately, at present, he has gained a reputation as a kid that can be teased into getting into trouble; other kids know they can play with his angry responses to teasing and get a "good show" in return. Your son is presently so controlled by his anger that he ends up suffering more than the teasing, he also gets punished for being caught responding with anger. He can't "win for losing".
His present age and being a boy make it hard to "back down" from the teasing. You need to help him find more emotional allies than just his anger. He has to learn new ways of simply moving through life without this much fear and anger; I say fear because usually the level of anger expressed by a child is directly related to the level of fear(s) he has.
As to your role in responding to the "teasers", I don't know if these kids are in his school alone or also in his neighborhood (or both). Regardless, I don't think you can accomplish much by scolding them or even politely appealing to them; I think it would further fuel the teasing by tossing in a "your mommy's got to fight your battles for you" opportunity to the "teasers". I would, however, consider talking with his teachers, confidentially, to make them aware of the situation and enlist their support and advice.
You need to learn different ways to relate to your boy, as whatever has gone on in the past has not brought about your feeling that you are helping him live a happier life. I truly believe that his finding a child therapist and your finding a good family therapist could help invaluably at this time. You both need the support and counsel of an objective, compassionate person who is in your corner. His unhappiness and inability to relate well to other kids is not going to suddenly disappear and at this age the opinions of his peers begin to mean much more than the opinions of "good old mom".
Please search for therapists who really suit the both of you and present this as a way for your boy to talk with someone who can help him be happier with his life. He should not be blamed or labeled with some disorder; he should finally have someone (and this probably can't be just you at this stage) who can empathize with him, form an alliance with him, hear his pain, anger and fears, and teach him successful ways to be happier with himself and in the presence of others. Good luck, and thanks for caring so much about your boy; I know it hurts when we can't take away the hurt for our kids.