He cheated

Hi, I need opinion from someone else. And I know that the answers are going to be mostly move on.
And I know that I should move on and that I don’t want to be with someone like him. But I still love him and I would like to get back with him when we are older. Because he needs to grow up.
The whole back story to my issue is in my first article here called He broke up with me for the second time.
But from that time things changed.
So, he broke up with me for the second time on 21 of May, he was drunk and didn’t say almost anything. I thought that he will contact me, that he’s sorry or something like that.
But he didn’t, week after I called him asking him if this is the end. And he said tgat yes. Without any emotion in his voice.
Two days after I saw him in a school for the end of high school (we are firm Czech Republic, he’s 19 I am 20) and I told him that I am sorry for what happened, but that I am not mad. That I want him to be happy and then I went away. Like I wanted some closure or something like that.
But I was expecting some reaction. But he haven’t say a word. He was just looking to the floor.
And then I saw him with one girl from our school. And it hurt. But maybe they’re just friends?
The next day we had some type of party with teachers. I needed to go home early because I went to the work the next day.
When I was walking home I saw her coming that way. I have not been able to understand why she was going to our class’s party when she’s not from our class.
Then I texted one of my friends, who knows her, if she has something with my ex. And suddenly she started to text me back. I don’t understand why my so called friend give her his phone.
But she sent me photo of her and my ex kissing.
Long story short we chatted for a little bit and discovered that the whole time he was with me (we got back together on 17 of April, they started texting on 5 of May) he was texting with her. The whole time he was saying to me that he wants to be with me, that he realized it when we were apart, that he wants to live with me and one day to get married. The whole time he was texting with her.
And she does not care. She knows that he was cheating on me, that he was lying to me and to her the whole time. But she just cared that we broke up on 21 and she slept with him on 23..
Then she told me that for her it’s closed chapter and if I want anything else to discuss to text her boyfriend. I told her that I already did it but he’s not answering.
She probably told him to text me back.
I told him that I am not mad. That I’m happy that I know what type of person he is. That I still care about him and whish him the best and that I hope that their relationship will last.
He just texted bye.

Do you think that this relationship will last? Is it just a rebound? Do you think he really lost all of his feeling for me? How could he even possibly be texting her and being with me? Is it possible for him to act so cold because she will read the messages?