Well the fact remains that this is a serious situation, and your comment: I wanted to get to know him more before making a choice but I think it is not fair on him and my boyfriend. Sums it up completely. The reality is it is not fair to your boyfriend for you to pursue this and get to know this other guy while still being with him.
I can understand and sympathize that you think you care for the ex and want to see where it goes. But it can not go anywhere as long as you both are in a relationship. Even if you could get to know him, while still being with your boyfriend it would not be a real evaluation since it would be clouded with guilt both of you would be feeling. The only way to really know is to end it with your boyfriend and then get to know your ex, otherwise all you will get to know is how it feels to cheat.
I know this is a difficult situation, and one you must make for yourself. I can not tell you what is the best route for you......I can only advise you to not disrespect your boyfriend or yourself by allowing this to continue.
You have two choices at this point. The first is to tell the ex how you feel and both of you end your relationships. The other is to take a step back and end this with your ex and move on with your boyfriend. You can not continue to talk with your ex while being with your boyfriend.........it simply is not fair to him or yourself.
If you decide to stay with your boyfriend you have to end contact with your ex. The reality is it is not fair to continue to talk with your ex when you are both in a relationship. He has a girlfriend and as such you must respect her and him by allowing them their own chance. This isn't about just you, or even him.......this is about all 4 of you. Your actions today can hurt 4 different people including yourself. I would strongly recommend that you sit down and do some serious thinking about what you really want. If your ex is that important to you, then far be it from me or anyone else to stand in the way of true love. But before you cross that bridge and burn it make sure this is what you really want. Love is about taking chances.........and the fact is you do not know your ex. Are you prepared to leave the one your with now for a chance?