Advice!!!

So my SO only gets jealous when a certain coworker talks to me. They all call me "shorty" because of my my height but when that one guy calls me shorty he gets vivid. And I've told him that guy does not interested me nor has he ever disrespected me in anyway. Is it me? Is it him? I'm getting very annoyed with the whole situation!!! Why does he get jealous over it? My coworker is like 50 !! And not my type !!! And I've never cheated or anything to have him doubt me.

I don't know

Anyone else have this problem? How can I get it threw his head there's nothing to be upset about?