This is one of those that the white is covoring too many of the indicators so you are not going to be able to tell just by his current pictures. A smutty buckskin and a grullo (black dun) can look identical.
Duns do have several indicators, however not all dun horses have all those indicators. These include dorsal, leg barring, shoulder shading, and a face mask. However - the smutty/sooty gene that causes countershading can create an illusion of these on a non-dun horse. On top of that - This horse has white where a lot of those indicators would be.
Do you have pictures of the parents (I don't trust registered colors - those can be wrong)? They can be a clue. Otherwise you will need a DNA test to know for sure.
Somersetfarms - Yes, grullo is a black-based horse with the dun gene - but the dun gene is also a dilution gene and will dilute the black coat down. I've seen grullos A very similar color. You can see some at the bottom of this page: Equine Color - Horse & Pony Color Genetics Information