Pictures would help ;)
Dun is caused by the dun gene. It lightens the coat and gives a true dorsal and often other primitive markings such as leg bars. Dun on black = grullo; dun on bay = bay dun, aka zebra dun, aka dun; dun on chestnut = red dun; dun on buckskin = dunskin; dun on palomino = dunalino...
Buckskin is caused by one copy of the cream gene on a bay horse. On a chestnut horse it would be palomino, on a black horse it would be smokey black (which may or may not appear as a normal black horse). Two copies gives perlino, cremello, and smokey cream, respectively. This does not cause true dorsals or primitive markings, it just dilutes the coat.
If his father truely is dun (I do NOT go by what they are registered as or what someone says they are, as that is very often incorrect), the it is POSSIBLE he is dun. If neither sire nor dam are true duns, then he cannot be. There are some true duns who's dorsal fades before the withers. There is also a dun test if you really wanna know, but the test is thought to be not 100% reliable due to the nature of it (hard to explain, but it's not like a test for cream or extension (black vs red)).