Is it worth it? Should i tell him? Please help!!! ASAP!?

I have known this guy for years. We have been best friends since we were 4 years old. We did everything together and he was my first friend. Because I lived in the middle of now where til i moved to my location now with neighbors. He is 2 houses down. Well anyways, we hangout all the time. We know everything about eachother, and always did. So we go outside in the summer and hangout with 2 other people. And a few years back we were playing truth or dare and we had our first kiss together. He won't idmit it but it happened. He hits on me, flirts, tells me about his problems and i do the same thing. He has a girlfriend of a year, on and off, and she doesn't even like him. Me and her a pretty good friends and we talk about him lots. She doesn't know how to break up with him. She's almost afraid. Who cares if he is 6'1 and over 150lbs. She's just scared. He likes her but he kinda wants to break upo with her also. What do i do?! tell them what they say about eachother?! idk! Me and him have romantic nights. I don't know if you wanna call it cheating or what. But, as i said they have been on and off. So about 2 months ago him and his girlfriend "broke up" he said "i'm not getting back with that ***** again". So he came up to my house and we were hanging out in my backyard. And after 2 hours of talking we got into the subjects friends with benefits. And after another 2 hours of talking about friends with benefits, 1 minute im sitting on the ground in criss cross position and the next minute i have a 6'1, 150lb. guy on top of me a 1/2 inch away from my face and for some reason i kept my eyes open and he say my expresstion and got off of me and grabbed his phone looked at it for 5 seconds and said o i gotta go and he walked to his house. And, i was kinda awkward the next day in math cl because we have igned seats and we sit next to eachother. It was awkward for me, i don't know if it was for him but i imagine it would be. So ever since that kiss when we hang out and there is constant flirting and and that stuff. Him and his girlfriend are pretty close to breaking up but, when they do should i say how i feel? because he has no idea how i feel. How do u think he will react? Will he feel the same way? Please help!? ASAP!!!