Need help in how to confront him with suspicions

So my SO and I live together and things were going good up until about 3-4 months ago. He started talking less and the frequency of sex went from every other day to like once a week. One night about a month and a half ago I went through his phone and found a slightly inappropriate conversation where he was calling a girl babe, sexy and saying how he wanted nothing more than a hug and kiss from her. I confronted him about it and the girl (before speaking to him of course) and both assured me that nothing was going on that they were just old friends. I asked him to never speak in that manner with women again and he agreed since he wouldn't be okay with me talking to men that way. Then somewhere along the way we decided that ttc was a good idea (not pregnant as far as I know yet). Every so often I take a quick look in his phone and last night found something confusing. He had texted a person (can't tell gender from the texts and there is no name saved) and they were unsure who was texting them so he responded with a username. There is nothing much to the conversation other than a how's it going but I am worried that he is trying to cheat. I made it clear to him that because of how distant he is becoming that I think he might be cheating and he was a little defensive (not sure if he was just a bit offended). But I don't know how to approach telling him about the texts with a username I saw. Any ideas? I don't want to come across as accusing him just yet, not until I can get more information.