Do you happen to know what his parents are? That would narrow it down quite a bit.
His dam was much like him, only a smidge lighter in color. Almost a buckskin? His sire died earlier this year, bit if I'm not mistaken the owner's said he was a Palomino. He had his half-sister on site, and she looked almost palomino paint.
It's hard to see, but the three are here. His dam, Atreyu, half-sister (?), and Bamber. I wish I'd have taken more pictures when I went to the go-see. :/