Guy already has girlfriend?

I've been texting this guy I know for a couple of weeks, and without him knowing I searched him on Facebook (he doesn't know I know his surname) and it says he's been in a relationship for nearly 2 years. I obviously feel incredibly confused by this as he's been texting me both lovely things, saying he wants to take me out on picnics and day trips and stuff, but he's also been saying sexual things quite a lot. I don't know if I'm potentially being used for sex and a 'bit on the side' or if he wants out of his relationship and hasn't told the girl yet. I've tried to broach the subject offhandedly without letting on I know anything but he's said he's not a cheat and he's a 'one woman man'. Do I tell him what I know or what?

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