Can you trust him again after that person has cheated?

We've had our ups and downs, in the beginning of our relationship we had people try to break us up. He was talking in a sexual manner to another chick on a dating site right after we got together. Years later I found him talking to his ex but it wasn't anything like he was cheating. Before we had trust for each other, but now I can't trust him. He won't even lay his phone down. He use to let me look at his phone no problem. Now it's like pulling teeth. I feel like he's cheating by talking to someone not having sex but I'm wondering if it's just my mind playing tricks on me.