Should I end it?

So this guy I went to high school with (but never spoke to) sent me a friend request on Facebook. I added him because I still knew him. Since then he’s been messaging me, two days, all day. I don’t always respond very fast, and when I don’t respond, he sends more messages. My friends said I shouldn’t respond at all. I have a boyfriend (who I absolutely adore). Facebook guy KNOWS this. Even if he hasn’t looked at my relationship status, I’ve brought my boyfriend up in conversation. Should I stop talking to him? If so, how can I do that in a nice way? Should my boyfriend know he messages me? Thanks!