I am currently 39 weeks pregnant and my stomach has gone unusually soft all over. I can usually feel the baby's back and behind but I can't seem to find them. What could this mean?

Have you called your Doctor? In the event this is something serious you shouldn't be wasting time on line.

He is still moving frequently and I am not having any pain or issues. The only thing I can think of is maybe he's moved into a posterior position.

Maybe so, but why wonder and worry? Call the Doctor.