Jennifer,
It's probably that he needs a little space and just doesn't want to hurt your feelings etc. If he's talking with another woman this can happen too, but if that's the road he's chosen, then you have to just let him walk down that path on his own.
I know it's hard to do, but if he's backing away, let him. Guys don't want girls to fight for them, but Girls want guys to fight for them.
Hopefully that's not what it is, but either way be supportive and respectfull and just stand your ground and put him in his place. Some guys need a good slapping around once and a while, like Mom used to do to him, and maybe he will come back around.
If not move on and he will most likely come back too you.
Something else you can do, is don't call him unless he calls you, no matter how long that takes, I garantee he will call you.