Your boyfriend, Choso wasn't a jealous type of guy. Hell he didn't really care much about other guys or girls flirting with you. He trusted you enough to know that you wouldn't cheat on him. He would usually stand by you or sit as you spoke to the other person. Since he was knew to relationships he didn't know what was flirting and just being kind. A main reason why he was calm.
However, it was different in this situation he was in. This guy flirting with you. He could tell he was flirting with you by the look on the man's face, tone, and his body language. Choso isn't stupid to realize that this man was trying to steal you away from him. He realized he was jealous. An emotion he wasn't used to feeling at all. Jealous? Why'd I be jealous? He thought to himself. He knew you wouldn't fall for this man. He just hated how close the guy was to you. Physically.
Choso stood right next to you. He draped an arm around your shoulder and glared at the guy. With his calm expression, it made it scarier.