Are we sure dam was a buckskin? Could she have been a bay dun?
He does appear to have that "slate/grey" color that many grulla's can have. Not all horses that carry the dun gene, show all the outward signs of dun. (zebra markings, strong dorsal stripe, shoulder barring, cob-webbing on forehead etc)
But, My guess would be brown.
"The greatest pleasure of a dog is that you may make a fool of yourself with
him and not only will he not scold you, but he will make a fool of himself too."