Do you think he is interested?

I have this guy friend, who i have known for a couple of years, and i think he is interested? But i am not sure if he is just flirting or is he actually interested. He is always there when i need him and we share everything together! He has a girlfriend who is very jealous of me. She says that he pays more attention to me than to her. One time when i was at a party with all of my friends and me and my guy friend were sitting next to each other and he asked me to dance, of course i said no because his girlfriend was there!, and these coincidences kept on coming! So i don;t know if he is just bored with his relationship and flirting with me or if he is interested. Please help! Thanks :)