Most Helpful Guy
When a guy starts to talk about their ex's frequently, he hasn't really moved on from that relationship (future dating tip). He doesn't know how to explain it to you once that desire to go back is set so he would probably act nice and tolerant (avoid creating arguments like saying it's okay, doesn't matter, etc) while trying to figure out how to end his current relationship to go back to his old one (and most likely he has been in contact with his ex while he's with you).
It's unfortunate you have to be one of many to experience that moment in your life and I understand why you aren't able to understand how guys think and act. I mean, that's why you're here right? I am saying this based on my speculation of what may have happened and how I interpret it, coming from a guy who's never been in a relationship.
Most Helpful Girl
I'm sorry that this happened. He was probably going through a low in his old relationship and wanted to escape and then there was you and it seemed like a good idea. Then he realised that he was quite happy with his ex and decided to go back. I wouldn't say that you were a rebound, since he left her for you, but I don't think we need to name it anyway.
The way he handled breaking up with you is very poor and disrespectful. It must be very painful and confusing for you, I am not surprised. Ideally, he should have had conversations about you wherein he should have told you that him and his ex are in touch and that he has doubts about you and him. Just leaving a box with your belongings on the doorsteps is what an utter coward would do.
Objectively, you should be glad that you got rid of someone like that, but I know that right now it's just hurtful. In a while, you will see that he isn't/wasn't worth any of the pain because the way he treated you is extremely low. As an aside, I think that leaving his ex for you five months ago was already pretty low. It seems like he seems to think he can just go and be with whoever he fancies to be with without taking into consideration that other people have feelings as well and deserve some respect.