1. Everyone has the right to a personal life. Yes, he shouldn't be cheating, but a partner doesn't have to be forced to share every piece of information either, and that should be respected.
2. The "freaking out" about you looking at the cell phone could easily have been a reaction over that you were rifling through it simply because you thought he was fooling around, which is a trust issue. See point #4.
3. Thinking that he is cheating based on several completely innocent messages from a girl late in the evening is not supportive enough. Is it the first time that a guy actually has girls as friends?
4. Yes, he's right about feeling guilty when you accuse him for no reason, it's feckin' bad to be wrongfully accused and so unworthy of being trusted.
5. And yes, you're seriously overreacting. If the message had said "Hey baby, want to come over for some nightly activities?", then this would've been appropriate. Right now you're grasping at thin air and acting as though it's radioactive and about to burn your hands off.
Chill out.