How to tell a guy you like him when he has a girlfriend?

I like this guy who is a year older than me but he has a girlfriend. I started talking to him and he started talking about how he has a mansion and 2 yachts and 6 jet skis. (He was kidding, he's a jokester). Was he trying to impress me? Also, he is really funny around me, another sign of impressing. He is very nice to me, when we were on skype chat he's like are you ok? I'm like yeah, i'm fine. He's like you sure? He always asks if I'm ok. Sign of flirting. Whenever he has to go he says gtg my job:) And when I say night he always says goodnight;). He has a girlfriend! Is he being friendly, like me, or just flirting with me? What should I do? Back off and let him come and chase me? How can I tell him that I like him?! Please help, thanks!