G.T.
Do a role play with him. Make a scenario that you know he's already been through such as kids making fun of the costume color. Tell him that you will be him and for him to say something to you to upset you like "I think the color of your costume looks stupid", then you answer with the response that you want to example to him. Pick some more things like losing at a game or some of the other things you've mentioned and that way you "show" him some ways to get through his little stressful episodes. Usually when kids get mad or say something we find concerning it's because they just don't have the skill to do or act any other way. Just teach him by example. Telling them what to do usually isnt enough, their little brains are able to wrap around certain concepts, showing is much better.