Mourning doves are terrible at building their nests in strategic places and it always seems like the babies fall out before they can fly. But despite not being the "sharpest tacks in the box," they are terribly attentive parents (they only lay usually two eggs at a time, so only one or two survive...and while they have side affairs, are usually mated for life or until one passes away...and share parenting). Especially if it's at the point where it can fly even a bit, it's probably just about ready to completely take off...like REALLY soon. I'd let them all do their thing. S/he is probably just hanging around near the nest for a day or two before getting up the guts to run off on his/her own!
(I watched many generations of mourning doves very closely).