As a man who tried reconciling with my wife before she submitted for divorce. I would ask you to let him know your feelings. Tell him, let him decide what he wants. Maybe his current girlfriend is only making him happy now to cover the memories you both have together.
Just be straight forward and tell him. Even if you fear regection. Tell him, release what’s been building up inside, whether his ego will boost from knowing you want him back or not, getting it off your chest and mind is the best thing you can do. I wouldn’t wait, you still have a chance. He’s engaged yes, but he’s not remarried yet. Only until the priest says, “Does anyone object or impose this marriage, forever speak now or forever hold your peace”, you still have a chance. But once he’s married, then and only then will I say, “maybe it’s time to move on”.
I hate people who don’t have hope and just say “let him go or move on.” They obviously don’t hear about the success stories and people coming back together. So keep hope, and good luck with everything.