Why is an ex-object of affection trying to make me jealous?

Well it kinda depends, did he like try to get with u while u were still underage and you said no? Or anything like that? If so he may be doing it jus to show off that he doesnt care. And if not he's just an and its better you arent with him anyway. Just tell him straight up to knock it off. If he dont listen then stop talking to him, as a teenager myself I can tell you we fall in and out of love easily, as soon as you get another boyfriend and give it a few weeks then you will have almost forgotten all about him.