He has a girlfriend but gets jealous if I talk to other guys.

So recently my "best friend" told me he's got a girlfriend whom he says he doesn't love. After that he told me his friend is coming from another town so I told him to set me and his friend up on a date AS A JOKE. So he was like: "Hey, me having a girlfriend doesn't mean that you just go out and..." he stopped there. So I asked him what he meant, he refused to reply. Later that night I was telling him about a guy that I kinda have a crush on so he asked if the guy looked better than him or is better than him to me. And I can keep on going with all the stories that's been happening whenever I'm around guys.

Important thing to mention that me and him had the "Feelings" talk 8 months ago and we made it clear that it's better if we stayed friends and he's made it clear that he's not in love with me. So what in the world is going on here?!
He has a girlfriend but gets jealous if I talk to other guys.
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