I don't think its necessarily too late for a sour, but you'll have to stick with a sour mash and no extra bugs in the carboy. Mash it really, really low, sour at least 1/3rd of the mash if not the whole damn thing, then boil it up and pitch a very fast, attenuative strain like WLP001.
You could also bottle half before fall and add some real bugs to the other half. Drink it next year
