How did he expect you to have a back for the quilt?
Although some people piece together the backing, most have a back made from all of the same fabric. The exception to this unless a turn down is added to the backing, which would be blocks or a design co-ordinating with the top.
Without the backing (just purchased) it would continue to remain a WIP.
The other way to look at the top, is that it's a completed top, waiting to be sandwiched for quilting. Put it in the completed (tops) pile.
Pam M