My boyfriends married male coworker is hitting on me and I don’t know what to do?

Met this guy at my boyfriend’s work party a few months back. He added me on social media which didn’t bother me initially. Now he will text me saying things like “hey beautiful” or that he would do things for me because he likes me and thinks I’m cute and silly.

I haven’t been responding to him lately and have only seen him in person 2 times in the last three months and he avoids me like the plague when we are in person. I haven’t told my boyfriend yet because I’m afraid of how he will react. Advice on what to say or do?
Updates
+1 y
This also JUST started in the last two weeks.
My boyfriends married male coworker is hitting on me and I don’t know what to do?
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