I think my boyfriend is cheating 💔

Me

I moved in with my boyfriend of 1 year a couple months ago. So today after dinner my boyfriend took his 9yo son to a baseball game, leaving me here. I was cleaning up and came across an old cell phone. Didn’t think much of it but it lit up when I moved it..not only was it on, but it was connected to the WiFi we just got a couple weeks ago..meaning it had been used recently. So I looked at it. My boyfriend is messaging a girl on Facebook, so his messages on his current phone are showing up on the old one. He was messaging her while I was having his son help me make dinner!! I read through and doesn’t look like he’s been talking to her for long..or doing anything physical. But the messages are very flirty. I can’t believe it! I completely trusted him. I packed all of my stuff and brought it to my car and am now waiting for him to come home :/ I am so heartbroken right now and I don’t even know what to say to him. I’m so involved with his son and mother and rest of his family I never once thought he would cheat on me! Help please I don’t know what to say :/