How do you teach a 5 year old to speak up when being bullied?

Over the past couple of months I have noticed a significant change in my son. He has stopped doing his work in class and the work that he does do is sloppy and/or unfinished. He's also been getting into trouble a lot at school and having an attitude at home. I've asked him time and time again if anything is going on that he wants to talk about and he says no. His teacher and I are both concerned that there is an underlying issue that he's not telling us about. He went from being the smartest kid in class to not doing any work, being well behaved and well mannered to ignoring people and not following directions. Well, today since he gets in trouble a lot more when he is at "specials" (art, music, pe, etc) I decided I would sit in and monitor his actions to see if I could see a problem. And boy did I!! He was pushed, hit, called names, and bullied throughout the last half of PE by 3 different kids. 2 of which are in his class!!! The third kid is in another class and only around him during PE. One of the little boys actually tried to slam my son's head into the wall. The PE teacher has 3 classes at once and she is the only teacher there so she didn't witness it happening. After he did this my son picked up his frisbee and went back to his area. Then later on the little boy went out of his way to get by my son and started pushing him and hitting him and trying to kick him. I couldn't stand it anymore so I ran over there and the boy froze. I told him to keep his hands to himself and to not touch y son again. Another kid pushed my son down and jumped on top of him. The teacher did see that and so that kid got in trouble. Then the third kid pushed my son away from him while they were supposed to be on the same team and told him to stay away from him. I asked my son later why he didn't tell the teacher when these kids were doing this and he said he just deals with it and sometimes when he tells the teacher they don't do anything about it and it sometimes makes the kids more mad at him. One kid even told my son that he wouldn't play with him because my son is white!!!!! I asked him how long this has been going on and my son said "a long time now" I asked him why he didn't tell me about it and he doesn't really give me an answer! I've always made sure that he knows I'm here for him and always kept an open line of communication, but a couple months ago is when he started withdrawing from talking to me. I'm assuming that is around the time this started getting worse. I'm so sad for him! I don't know exactly how to get him to open up more and tell someone when this is happening. Any ideas??

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