How do I handle my bfs anger issues?

ca

So my boyfriend has anger issues, he’s never been abusive in any way and it’s something he’s actively working on. He doesn’t yell at me really bad or anything like that, but sometimes he’s just very critical of very small things. For example yesterday we were going to the grocery store to buy some things to make dinner for his parents. I told him I didn’t care what we got for dinner I was hungry for anything, so we should just make the preference of his parents or him. He got really mad at that because he thought I wasn’t being honest and was being sassy. (I wasn’t though). Anyway he looked like he was turning red and he just closed his eyes and started breathing really slowly, looked like I was about to get yelled at really really bad and he was just trying to calm himself so he wouldn’t do that. I mean I appreciate him making an effort to work on his anger, he knows he has a problem and I’m trying to help as well. But idk that just made me so nervous like he was about to explode, what do I do when he does that? Should I leave the room? Should I say something encouraging? Do I play music? I appreciate him trying but I can’t help but feel kinda nervous