This isn't a case of workplace aggression, just some guy I work with who only yells to communicate. He's not mad ever, he just only talks in a super loud voice. Everyone tells him to talk quieter and he doesn't. It gives me a headache when he comes over to my desk and starts yelling at me.
You need to simply ask him why he yells. He may not even realize it. I've been told that I yell sometimes when I'm not intending to, even in normal conversation. I have a hearing problem and some very bad tinnitus. It could be that.
You're too loud. You're giving me a headache. Talk quieter or get or go somewhere else.
No reason to sugar coat. If others have been telling your coworker this and they're still too loud then they probably don't realize it's serious. Being rude will help them understand it's a real serious problem. Also, the responses above aren't actually rude enough to cause any disciplinary actions.