You could try an open relationship. Meaning that both of you date each other, but on occasion you both go out with other people. There's nothing wrong with this as long as you're honest and don't try to hide anything. Dating two people at once is sure to keep things interesting.
Look at it this way:
If your boyfriend is like the average human male, he will suffer from the male disease (also known as Being Full of Shit), and he will feel the need to prove he is a better boyfriend then your second boyfriend (or girlfriend if you choose that). He will try to one up them and make the dates he takes you on really interesting.
I don't buy into the whole monogamy thing. Stupid church rules is what that is. Honesty is key. As long your honest, it's technically not cheating.
Talk to him about it. It can't hurt to try. Good luck
Here are two possible reasons you lost interest. You subconsciously realized the two of you aren't compatible, or you were disappointed when you lost the happy squealy butterfly feeling you had when you first got together.
If it's the first one, then you should find someone else. You're just falling for him again, and it's not going to work.
If it's the second one, it depends. Different people need different things out of a romantic relationship. Some people like to build a romance slowly out of a close friendship. They're happy in a very close, trusting relationship with the same person with the rest of their life. Some people are only happy with the new-relationship excitement. Once that goes away, they move on to a new person and more excitement. Those are obviously extremes--there's a full spectrum of preferences in-between with pros and cons for each one. Before you decide whether you get back together with him, you should try to get an idea of where you are on that spectrum.