Just because he has been with you for eight months doesn’t mean that he is invested in your relationship. I know a guy who has been with a girl for about seven months, and she is clearly a rebound for him. He has even told me that he doesn’t want a future with her–he enjoys the time they spend together and she fills a void in his life, but he knows she is not the one for him.
The fact that you felt it was necessary to look at his phone proves that your gut is telling you that something is off. And when you looked, you found proof that something is. Don’t try to rationalize this. He’s not over her and he’s not giving himself fully to you. This is probably a really awful thing to hear, and I’m sorry, but you need to protect yourself. He would not be talking with his ex and saying he still cares for her if he was over her.