Should I leave him?

So I am 19 and I am seeing a guy who is 39. I personally don't care about age too much. We have been talking a lot and I spend the night with him almost every night. I have even met his mom and ahe is super nice and seems to really like me. Well he recently told me he loved me and I didn't say it back because I wasn't sure if I did yet, well I recently told him I felt the same way. Fast foward to tonight, he gets a message on his phone and I am sitting next to it and see the message. Nothing bad or anything, so I just read it. He is asleep now. The message is from his ex, and I scroll up a little and see him saying that he loves her and have a great day and stuff, which she doesn't really say back except once, but it made me have a mental breakdown because I thought I could trust him and he is still talking to his ex and we said we were only going to see each other... I am guessing most of you guys are going to say I should leave him? I really don't want to...