I like your guess. I do see draft in him, for sure.
IT's distantly possible that he is a mustang/draft cross, too. There were some draft stallions released into wild horse herds back in the 20's or earlier to help breed "free" farm horses of a good draft type. I think in particular in the Oregon herds, but please correct me if I am wrong on that.
I don't know much about breeds but to me he looks like he has some sort of draft in him. To me he looks big in a drafty way rather then a QH way. Also the feathering on the fetlock makes me think this. I could see appy in there because of the colour but I'm not sure.
he has the sparse mane, mottling and white sclara of an appy. I find the majority of appys have lots of quarter horse blood in them. He has big feet and somthing about the way he is built that says draft to me. on a different note, his mane is silver. is this an effect of roaning?