Since you broke up with him, he has every right to see other people. But you also have every right to be jealous because of it. People break up because something isn’t working. Sometimes they still have feelings for each other. It’s really not that uncommon.
And maybe he isn’t happy. Maybe the dates he’s going on are just to fill the gap you left behind when you broke up with him. People fake being happy all the time. He’s probably trying to move on, as people want to do. Getting out of a relationship is tough. It leaves holes where you don’t expect them. It’s easier to try to fill them with something, like dating, than nothing at all.
My real question is: if you still “have a lot of feelings for him” then why did you break up with him in the first place? You should probably think really hard about that and decide if you’re just jealous or if you made a big mistake.
You should probably talk to him.