Actually, he is a smutty buckskin. The smutty gene will give that dirty coloring on his body and will sometimes make some striping. I have a bay filly who has those markings on her shoulders, she is a smutty bay. Her dam is carrying the smutty gene as can be seen by the dorsal stripe down her back. The filly has that stripe too.
My buckskin stallion was registered as a dun because of his dorsal stripe, but he too is just a smutty buckskin. He is the sire of my filly Marbles who is a true brindle. Still waiting on more results with her, but so far it seems that she will be able to reproduce herself. I think the smutty gene is at play with her too.