@JJCOOL007,
He might be cheating, but he might not. You are basing your assumptions on the fact that he won't let you see his phone. The only reason you want to see his phone is that you don't trust him. If you don't trust him to the point of wanting to check up on him, then I have news for you. Your relationship is over.
Trust is such a vital part of any relationship. I would never, ever ask my wife if I could check her phone. I trust her completely. And she has never asked to check mine. The two of you have trust issues dating back to when he cheated. So he should be going out of his way to avoid anything that would make you wonder if he is cheating. He doesn't seem to be taking that route, but rather keeps talking with this girl even though he knows it bothers you.
I think the two of you need to sit down and talk. The relationship can be saved, but both of you will have to change. You need to get to the point where neither of you needs to ask to see the other's phone because you trust that even if you are chatting with someone of the opposite sex, you know it is innocent. When you get to that point, you'll know you have a rock solid relationship.
Now that I have rambled on way too long to make this point, I wish you good luck.