I've been on the other side, in a slightly similar situation. I wasn't getting my emotional needs met (he was ignoring me, texting/calling other girls, hanging out with his friends all time, being overly critical and could never find anything nice to say to me), yet we had been together for 3 years. I couldn't forget about how close we were, how many things we had shared together, how many difficulties we had faced and overcome... I wondered if it was just a phase since he was going through a lot of difficult, stressful circumstances... so I thought I would wait it out, stand by him and just suck it up when he treated me badly.
Well, I met another guy; neither of us had any intentions of anything, but before I knew it, we were very interested in one another. He treated me well, listened to what I had to say, respected me, valued me, noticed my strengths, encouraged me instead of criticizing me.... We had all the important things in common, life goals, interests, values (very surprising since I have a wide range of interests)... I felt completely torn in two. He wanted to be with me; he wanted something for real and official. Yet, I had 3 entire years of being with my boyfriend - I can't ignore that. It wasn't a fair choice. 3 years, best friends, closer to him than to anyone else in my life (and same for him) with one guy - compared to a month of just talking mostly online with the other.... The obvious choice is my boyfriend.
My boyfriend ended up finding out that I had been talking to this guy, we had a huge fight, I thought he was leaving me (even though I had never actually done anything with the guy; we just talked).... but we talked everything over and worked things out. He wanted to fix our relationship because he loved me and didn't want to lose me; of course I did too. So I told the other guy, and had to cut off contact with him because I felt too distracted to just stay friends with him. He understood me so well, our personalities complemented one another's so perfectly... I could not talk to him as a friend. So I ended everything with him.
Was I using him? No. Not in the least. Did I get rid of him because I could care less about him? Absolutely not. I had to do the mature, responsible thing, and honor my commitment to my boyfriend and to our deep friendship. It wasn't fair to my boyfriend to keep a friendship with a guy who wanted more with me, and it wasn't fair to the other guy to always be reminded how well we complemented and understood each other, yet he can't have me. How torturous and painful.
Sure, the guy could be using you - but, he may have found himself in the exact same type of situation. It's just not a fair choice between someone you're committed to and have been with for a long time vs. someone you just met. There's a good chance that what you feel for the new person is purely hormones anyway.
So for his sake, respect him and don't try to contact him or stay in his life. It'll be a constant shadow and reminder in his relationship. If it doesn't work out with them, he won't feel very appreciative that you kept bothering him and trying to 'win' him. He would likely appreciate it more if you respected him and gave him his space to work out his life for the best of everyone involved.