I'm seeing leg barring (prominent on the front leg) and faint shoulder stripes. Both are indicative of dun. I'd bet my bottom he's probably a dunskin (buckskin + dun). Like I said before, you cannot get a dun out of non-dun parents...and if the foal was truly a grullo then either your mare or that stallion has to be dun...and it's pretty obvious it isn't your mare.
EDIT: Just looked up his pedigree. If I've found the correct Sterling Shiver, his sire was a grullo (black dun).