I Need help

So I'm talking to this guy for like 4 months now & he likes me I like him we've  had sex. he's a couple years older than me but I recently found out he has a girlfriend and I've met his parents & brothers and he told them I was his girlfriend I was confused because he never asked me to be his girlfriend. they all seem to like me but I don't know if I should tell him that I found out he has a girlfriend or what should I do am I the side or she is ?