I will wait till he finish his first molt. Cockerels will show them true color after they first molt 18 months of age.I have only seen parasitic white grow out from the base of his tail, up the shaft of the tail feather. Not just a spot like this.
His underfluff at the base seems to be a nice dark grey.
I am sure @Chooks man will have a good answer for you.
Chooks man