Why is he acting jealous?

I met a guy and we became really good friends. We have a really flirty relationship and I started falling for him. I'm really shy so I never told him how I felt about him, I guess I was waiting on him to make the first move. He hooked up with a friend of mine so I figured I was just a friend to him. I started dating a new guy and now my guy friend is acting weird. He makes a huge point to tell my boyfriend if we hang out together. He keeps making little comments about how he's gonna steal me away from my boyfriend and he wants us to run away together. He gets upset if I bring my boyfriend to hang out when all my friends are together. He refuses to admit to me that he hooked up with my friend and changes the subject any time I bring up my boyfriend. I can't tell if he's being serious or if he's just jealous cause he's not the most important guy in my life any more.