Here is a little color lesson....
A buckskin is basically a bay that has a single dilute cream gene which acts on the color by making it 'lighter' (just like it would make a chestnut into a palomino). If the 'bay' were to have two dilute genes it would end up being a perlino (and the chestnut would be a cremello).
The "dun" factor also dilutes the color but it lightens from the knee area up towards the backbone, leaving color on the backbone, or what we call a dorsal stripe. So in order for a horse to genetically be a dun, it must have a dorsal stripe.
Some "bay" horses can have a single dilute cream gene AND the dun factor, making it a dunskin.
So since your horse has a dorsal stripe he's definitely not a buckskin. He could be a dun or a dunskin.
Either way, he's adorable!