Ways to get my bf to express his anger without blowing up?

Control: Unfortunately you can't get him to change his behavior unless he is willing to. As a psychiatrist I have learned that i can lead the way, but it is totally up to the person if they want to follow or not. Discuss it a time when things are calm. Encourage him to consider an anger management class or therapy. Explain how it impacts you. But don't get your hopes up.