Yeah, it's most likely that he's cheating. A slim chance that he had a brainfart, but I doubt it. Look, I wouldn't take any chances. I'd confront him, give him hell, tell him you know he's cheating, (but don't tell him what you know) and tell him to just admit it. Regardless of whether he tells you or not, I say you should just leave. The trust is already broken, and trust is not one of those things that repairs. You could also bypass this whole thing entirely and just leave, and offer your explanation some other time.
Bottom line is, yeah I think he's cheating, the trust is gone, and you should leave.
Well obviously that was someone else name(sounds like a girl name)but we do not know who is that person.Yes big chance he is cheating but not enough proof to accuse him.Try to figure out if his family members or female friends has that name.
If I were in your shoes I would do one of two things.
You could teasingly say something like, whos Simone? and see how he reactsand explain that it was in the text.
Or you could just take it at face value and give the situation the benefit of the doubt that it was a typo. I don't think this is enough info to definitavely say he is cheating on you. You would have to look for other signs.