the best thing is not to sweat him. if suddenly stop calling, he's going to wonder why you stop calling him.
i don't understand young men today.
maybe he's going through something and he's trying to sort it out.
he could be pulling away from you b/c he doesn't feel the same. do you think he found someone else and he's afraid to hurt you.
that is really weird that he would change over night, theres a reason why, you just don't know what it is.
i left a guy who was confused throughout our 5 year relationship. he put me through so much with him saying he love me one day, and feeling like he wanted to date other women the next.
try to concentrate on something else other than him. if he doesn't want to answer your call, then forget him.