Seems simple enough to just figure out with him at what volume level it becomes a problem for you. After you've given him his new volume 'limit', tell him that if he turns it up higher than that, you're going to have to talk to the boss. Explain that you actually care about doing a good job and are an honorable enough person to do the best job you can since someone is paying you to do it. You and your clients shouldn't suffer due to your coworker's lack of honor and/or self-discipline.