Yes I know the mothers of his chicks as they are in my flock. They are relatively easy to find and put into kippen (gold laced, black mottled, partridge and cuckoo) except for the wyandotte who has red or dark buff lacing on top of a white background, but with a grey head. The white was told to me here that it was splash blue.
Then you could try working through each mother, with various possibilities for the father, to try to get the right chick. You probably already tried that, but now try assuming that everywhere the rooster's color requires a dominant gene, he also has a recessive gene (one that matches the hen, or one of the other options for that gene.)
Of course, that makes lots of options to try...