Have you ever seen a baby play with a toy, then lose interest, and then start playing with another toy, then when it sees someone else playing with it, it thinks it should have it back, as if it owns the toy, whether or not it does? Well that's how your situation sounds. It wants the other toy but also wants the one its playing with. Its a funny little thing, huh? But the fun part is making him jealous back. Hehehehehe...
For the first while, he will get jealous seeing you with a different guy, but later on he will stop. Even though you guys are technically broken up, but emotionally he may still feel attached to you. So yeah..
There could be two reasons. The first one could be that you broke up with him and although he is going out with someone ells he still wants you and therefore are jealous because you are with someone ells. The second reason could be that he broke up with you and he like the idea of him being seen with other woman but don't like that you are also capable of getting attention from other guys
He still likes you and has feelings for you it the same way with my ex bf and my ex bf still likes me he asked me back out but I said no because I was dating some else and he got mad at school cause I was hanging with a bunch of guy that were my bfs friends and of course my bf was there and my ex got mad and punched the locker.
It is always hard after a break up. Even though things did not work out between the two of you, there are still feelings involved. It is hard to see someone you care about with another person. In time, it will get easier, and you won't have to worry about it anymore.