Not much of advice but maybe you keep on going out with him. Like going to places there will bring your old happy memories when you were together. Don’t tell him you want him back yet. I think you need to plant the idea of you want to get back together instead of telling him verbally. I know it’s a bit difficult to play mind games. By planting him good old memories might make him think about giving yout relationship second chance. Once you get the signal, that you desire, ask him to break up his current rebound relationship. Hope it helps!