Have you considered clearing the room, leaving only you and the student left in the room to try to resolve his anger? Sometimes, the "sudden" change of environment is enough to spark some conversation IF you have someone you can call to manage the rest of the kids. Arrange it in advance. The element of surprise (when the room is emptied) might be enough for him/her to see that you are concerned about the safety of both the student AND the rest of your kids.