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.